Adds Contributor guidelines for developers

Takes the guidelines already published on ThinkUp Documentation
and boils them down for a quicker read.

Links back for those who want to read up fully on the subject.

References:

- http://thinkupapp.com/docs/contribute/developers/devfromsource.html
- http://thinkupapp.com/docs/contribute/developers/pullrequestchecklist.html

Closes #1384
Merge pull request #1434 from randomecho/1384-add-contributor-guidelines

Adds Contributor guidelines for developers [ci skip]
Closes #1384