Add client guide.

[mofirouz]

This work needs to be submitted against the nakama-docs repository so we can publish it directly on the website.

Follow this guide format: https://github.com/heroiclabs/nakama-docs/blob/master/docs/android-java-client-guide.md

This should be considered separate additional documentation to Cocos2d-x JS docs.

[Dimon4eg]

Done.