Improve README.md (#5)

* Add badges

* Improve README.md

Author: nakamura-to

README.md

@@ -1,17 +1,17 @@
 Doma CodeGen Plugin
 ===================
 
-![Java CI with Gradle](https://github.com/domaframework/doma-codegen-plugin/workflows/Java%20CI%20with%20Gradle/badge.svg)
-
 Doma CodeGen Plugin is a gradle plugin.  
 It generates Java source files and SQL files from Database.
 
+[![Java CI with Gradle](https://github.com/domaframework/doma-codegen-plugin/workflows/Java%20CI%20with%20Gradle/badge.svg)](https://github.com/domaframework/doma-codegen-plugin/actions?query=workflow%3A%22Java+CI+with+Gradle%22)
+[![Google Group : doma-user](https://img.shields.io/badge/Google%20Group-doma--user-orange.svg)](https://groups.google.com/g/doma-user)
+[![Twitter](https://img.shields.io/badge/[email protected]?style=flat)](https://twitter.com/domaframework)
+
 How to use
 ----------
 
-First, see [Gradle Plugin Portal](https://plugins.gradle.org/plugin/org.seasar.doma.codegen).
-
-The basic usage example is as follows:
+The basic build.gradle example is as follows:
 
 ```groovy
 buildscript {
@@ -57,6 +57,13 @@ To generate all files, run `domaCodeGenDevAll` task:
 $ ./gradlew domaCodeGenDevAll
 ```
 
+See also [Gradle Plugin Portal](https://plugins.gradle.org/plugin/org.seasar.doma.codegen).
+
+Sample Project
+--------------
+
+- [codegen-sample](https://github.com/domaframework/codegen-sample)
+
 Gradle Tasks
 ------------
 
@@ -238,8 +245,3 @@ domaCodeGen {
 ```
 
 The Doma CodeGen Plugin uses [Apache FreeMarker](https://freemarker.apache.org/) to process the template files.
-
-Sample Project
---------------
-
-- [codegen-sample](https://github.com/domaframework/codegen-sample)