Add a Gitter chat badge to readme.md

### wikibook/androidndk now has a Chat Room on Gitter

@wikibook has just created a chat room. You can visit it here: https://gitter.im/wikibook/androidndk.

This pull-request adds this badge to your readme.md:

[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/wikibook/androidndk?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)

Happy chatting.