add link on realisation api for java client

doc/api/README.md

@@ -88,3 +88,4 @@ When listing resources you can pass the following parameters:
 + [php-gitlab-api](https://github.com/m4tthumphrey/php-gitlab-api) - PHP
 + [Ruby Wrapper](https://github.com/NARKOZ/gitlab) - Ruby
 + [python-gitlab](https://github.com/Itxaka/python-gitlab) - Python
++ [java-gitlab-api](https://github.com/timols/java-gitlab-api) - Java