Group & Organize Contrib Readme, Add Table of Contents 

#### SUMMARY: 
* Reordered and grouped according to similarity of topic
* Added some horizontal rules to break up content
* Created Table of Contents for Contributing contents

#### NOTES: 
Hey! I'm a new contributor, so please let me know if I should open an issue or go through some other workflow to help keep everyone's sanity intact :D 

I made some minimal decisions about ordering of contents -- but this was almost totally arbitrary, so please let me know if I'm really off-base, or if there's a Most Desired Order.