Style guides, coding conventions for Flutter Engine

While we have a [style guide](https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo) for the flutter/flutter repo, we should also document style/coding conventions for the flutter/engine repo. 

This should be linked from the wiki and the engine's CONTRIBUTING.md file, and cover the following languages:
* C++
* Objective-C / Objective-C++
* Java