docs: Added specification for OpenAPI

Specified that 2.x.x branch is for Swagger and OpenAPI version 2, and that branch 3.x.x is for OpenAPI version 3.

Resolves: #9475
See also: #9588

Author: josh31416

README.md

@@ -179,10 +179,10 @@ 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 8 runtime at a minimum):
 
 ```sh
-# Download current stable 2.x.x branch
+# Download current stable 2.x.x branch (Swagger and OpenAPI version 2)
 wget https://repo1.maven.org/maven2/io/swagger/swagger-codegen-cli/2.4.12/swagger-codegen-cli-2.4.12.jar -O swagger-codegen-cli.jar
 
-# Download current stable 3.x.x branch
+# Download current stable 3.x.x branch (OpenAPI version 3)
 wget https://repo1.maven.org/maven2/io/swagger/codegen/v3/swagger-codegen-cli/3.0.16/swagger-codegen-cli-3.0.16.jar -O swagger-codegen-cli.jar
 
 java -jar swagger-codegen-cli.jar help