docs: Maven repo missing 3.x.x versions

Download url for current stable of the 3.x.x branch was missing in the prerequisites section of the README.
Without this specification the user may not realize their is a newer version available, specially those who don't want to clone the project and build it.

Resolves: #9475
See also: #9588

Author: josh31416

README.md

@@ -179,8 +179,12 @@ 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 8 runtime at a minimum):
 
 ```sh
+# Download current stable 2.x.x branch
 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
+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
 ```