Adjust the Become a Contributor page copy.

Based on feedback, adjust the Become a Contributor page copy to be more
descriptive and accurate.

Completing https://trello.com/c/Uc4yJUsQ

Based on:

![](http://i.cwebber.net/Become_a_Contributor_-_Chef_Supermarket_2014-06-25_09-32-06_2014-06-25_09-37-51.jpg)