Add CONTRIBUTING.md

CONTRIBUTING.md

+# Contributing Guidelines - Rocket.Chat.Android
+
+Great to have you here! Here are a few ways you can help make this project better!
+
+## Reporting an Issue
+
+[Github Issues](https://github.com/RocketChat/Rocket.Chat.Android/issues) are used to track todos, bugs, feature requests, and more.
+
+## Setting up a development environment
+In case you're interested in playing around with the code or giving something back, here are some instructions on how to set up your project:
+
+### Pre-requisites
+1. [Android Studio and SDK Tools](https://developer.android.com/studio/index.html).
+2. Clone this repo:
+```
+git clone https://github.com/RocketChat/Rocket.Chat.Android
+```
+3. Build the project.
+
+### Code style guide
+
+Before submitting a PR you should follow our [Coding Style](https://github.com/filipedelimabrito/Rocket.Chat.Android/blob/develop/CODING_STYLE.md).