add versioning section to readme

Author: frantuma

README.md

@@ -2,16 +2,15 @@
 
 [![Build Status](https://img.shields.io/jenkins/s/https/jenkins.swagger.io/view/OSS%20-%20Java/job/oss-swagger-core-master-java-8.svg)](https://jenkins.swagger.io/view/OSS%20-%20Java/job/oss-swagger-core-master-java-8)
 
-- Master (2.4.1): [![Build Status](https://img.shields.io/travis/swagger-api/swagger-codegen/master.svg?label=Petstore%20Integration%20Test)](https://travis-ci.org/swagger-api/swagger-codegen)
+- Master (2.4.4): [![Build Status](https://img.shields.io/travis/swagger-api/swagger-codegen/master.svg?label=Petstore%20Integration%20Test)](https://travis-ci.org/swagger-api/swagger-codegen)
 [![Java Test](https://img.shields.io/jenkins/s/https/jenkins.swagger.io/view/OSS%20-%20Java/job/oss-swagger-codegen-master.svg)](https://jenkins.swagger.io/view/OSS%20-%20Java/job/oss-swagger-codegen-master)
 [![Windows Test](https://ci.appveyor.com/api/projects/status/github/swagger-api/swagger-codegen?branch=master&svg=true&passingText=Windows%20Test%20-%20OK&failingText=Windows%20Test%20-%20Fails)](https://ci.appveyor.com/project/swaggerhub-bot/swagger-codegen)
 - 3.0.0:  [![Build Status](https://img.shields.io/travis/swagger-api/swagger-codegen/3.0.0.svg?label=Petstore%20Integration%20Test)](https://travis-ci.org/swagger-api/swagger-codegen)
 [![Java Test](https://img.shields.io/jenkins/s/https/jenkins.swagger.io/view/OSS%20-%20Java/job/oss-swagger-codegen-3.svg)](https://jenkins.swagger.io/view/OSS%20-%20Java/job/oss-swagger-codegen-3)
 [![Windows Test](https://ci.appveyor.com/api/projects/status/github/swagger-api/swagger-codegen?branch=3.0.0&svg=true&passingText=Windows%20Test%20-%20OK&failingText=Windows%20Test%20-%20Fails)](https://ci.appveyor.com/project/swaggerhub-bot/swagger-codegen)
 
 
-[![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.swagger/swagger-codegen-project/badge.svg?style=plastic)](https://maven-badges.herokuapp.com/maven-central/io.swagger/swagger-codegen-project)
-[![PR Stats](http://issuestats.com/github/swagger-api/swagger-codegen/badge/pr)](http://issuestats.com/github/swagger-api/swagger-codegen) [![Issue Stats](http://issuestats.com/github/swagger-api/swagger-codegen/badge/issue)](http://issuestats.com/github/swagger-api/swagger-codegen)
+[Maven Central](https://maven-badges.herokuapp.com/maven-central/io.swagger/swagger-codegen-project)
 
 :star::star::star: If you would like to contribute, please refer to [guidelines](https://github.com/swagger-api/swagger-codegen/blob/master/CONTRIBUTING.md) and a list of [open tasks](https://github.com/swagger-api/swagger-codegen/issues?q=is%3Aopen+is%3Aissue+label%3A%22help+wanted%22).:star::star::star:
 
@@ -23,6 +22,55 @@
 
 :notebook_with_decorative_cover: The eBook [A Beginner's Guide to Code Generation for REST APIs](https://gumroad.com/l/swagger_codegen_beginner) is a good starting point for beginners.
 
+## Versioning
+
+**NOTE:** version 2.X (`io.swagger`) and 3.X (`io.swagger.codegen.v3`) have **different** group ids.
+
+2.X and 3.X version lines of Swagger Codegen are available; 2.X (`master` branch) supports Swagger/OpenAPI version 2,
+while 3.X ([`3.0.0` branch](https://github.com/swagger-api/swagger-codegen/tree/3.0.0)) supports OpenAPI version 3 (and version 2 via spec conversion to version 3).
+[Online generator of version 3.X](https://github.com/swagger-api/swagger-codegen/tree/3.0.0#online-generators) supports both generation from Swagger/OpenAPI version 2 (by using engine + generators of 2.X) and version 3 specifications.
+
+
+**NOTE:** this document refers to version 2.X, check [here](https://github.com/swagger-api/swagger-codegen/tree/3.0.0) for 3.X.
+
+
+### Swagger Codegen 2.X ([`master` branch](https://github.com/swagger-api/swagger-codegen/tree/master))
+
+Swagger Codegen 2.X supports Swagger/OpenAPI version 2.
+
+group id: `io.swagger`
+maven central (maven plugin): https://mvnrepository.com/artifact/io.swagger/swagger-codegen-maven-plugin
+
+dependency example:
+
+```
+<dependency>
+    <groupId>io.swagger</groupId>
+    <artifactId>swagger-codegen-maven-plugin</artifactId>
+    <version>2.4.4</version>
+</dependency>
+```
+
+### Swagger Codegen 3.X ([`3.0.0` branch](https://github.com/swagger-api/swagger-codegen/tree/3.0.0))
+
+Swagger Codegen 2.X supports OpenAPI version 3 (and version 2 via spec conversion to version 3)
+[Online generator of version 3.X](https://github.com/swagger-api/swagger-codegen/tree/3.0.0#online-generators) supports both generation from Swagger/OpenAPI version 2 (by using engine + generators of 2.X) and version 3 specifications.
+
+group id: `io.swagger.codegen.v3`
+maven central: https://mvnrepository.com/artifact/io.swagger.codegen.v3
+
+dependency example:
+
+```
+<dependency>
+    <groupId>io.swagger.codegen.v3</groupId>
+    <artifactId>swagger-codegen-maven-plugin</artifactId>
+    <version>3.0.7</version>
+</dependency>
+```
+
+
+
 ## Overview
 This is the Swagger Codegen project, which allows generation of API client libraries (SDK generation), server stubs and documentation automatically given an [OpenAPI Spec](https://github.com/OAI/OpenAPI-Specification). Currently, the following languages/frameworks are supported:
 
@@ -109,12 +157,12 @@ Swagger Codegen Version    | Release Date | OpenAPI Spec compatibility | Notes
 If you're looking for the latest stable version, you can grab it directly from Maven.org (Java 7 runtime at a minimum):
 
 ```sh
-wget http://central.maven.org/maven2/io/swagger/swagger-codegen-cli/2.3.1/swagger-codegen-cli-2.3.1.jar -O swagger-codegen-cli.jar
+wget http://central.maven.org/maven2/io/swagger/swagger-codegen-cli/2.4.4/swagger-codegen-cli-2.4.4.jar -O swagger-codegen-cli.jar
 
 java -jar swagger-codegen-cli.jar help
 ```
 
-For Windows users, you will need to install [wget](http://gnuwin32.sourceforge.net/packages/wget.htm) or you can use Invoke-WebRequest in PowerShell (3.0+), e.g. `Invoke-WebRequest -OutFile swagger-codegen-cli.jar http://central.maven.org/maven2/io/swagger/swagger-codegen-cli/2.3.1/swagger-codegen-cli-2.3.1.jar`
+For Windows users, you will need to install [wget](http://gnuwin32.sourceforge.net/packages/wget.htm) or you can use Invoke-WebRequest in PowerShell (3.0+), e.g. `Invoke-WebRequest -OutFile swagger-codegen-cli.jar http://central.maven.org/maven2/io/swagger/swagger-codegen-cli/2.4.4/swagger-codegen-cli-2.4.4.jar`
 
 On a mac, it's even easier with `brew`:
 ```sh
@@ -259,7 +307,7 @@ java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate \
 ```
 (if you're on Windows, replace the last command with `java -jar modules\swagger-codegen-cli\target\swagger-codegen-cli.jar generate -i http://petstore.swagger.io/v2/swagger.json -l php -o c:\temp\php_api_client`)
 
-You can also download the JAR (latest release) directly from [maven.org](http://central.maven.org/maven2/io/swagger/swagger-codegen-cli/2.3.1/swagger-codegen-cli-2.3.1.jar)
+You can also download the JAR (latest release) directly from [maven.org](http://central.maven.org/maven2/io/swagger/swagger-codegen-cli/2.4.4/swagger-codegen-cli-2.4.4.jar)
 
 To get a list of **general** options available, please run `java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar help generate`