Improve CONTRIBUTING guide.

Add some line breaks(better when reading in the editor) and use markdown syntax for ordered lists.
Also, added a title and the simple_form logo.

## Before:
![screenshot 2014-01-28 23 20 08](https://f.cloud.github.com/assets/177525/2026210/0ad60598-888c-11e3-93a2-f797d6ca51bc.png)

## After:
![screenshot 2014-01-28 23 20 28](https://f.cloud.github.com/assets/177525/2026212/1274a926-888c-11e3-8270-cf7de50d35ac.png)