Add CONTRIBUTING.md, close tiimgreen#5

Author: tiimgreen

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# Contributing
+
+1. Fork it (http://github.com/tiimgreen/github-cheat-sheet/fork)
+2. Commit your changes
+3. Push changes
+4. Create new Pull Request
+
+## Styling
+
+- Use h2 for header:
+```markdown
+## New Feature
+```
+- Add examples wherever possible
+- Use `bash` styling for all git commands:
+    ```bash
+    $ git commit -m "Message"
+    ```
+- Add links to Git docs or GitHub docs wherever possible