Add file developer-workflow.md

- describes workflow of developers, including Terminal commands, footnotes, links to web
- addresses [Pivotal Tracker issue 76274824](https://www.pivotaltracker.com/story/show/76274824)
- almost the same as file previously pushed, then deleted, except that it is in its own branch so it can be merged separately from admin-workflow
- now includes:
  - suggestions to get SSH key, examples show SSH (instead of HTTPS)
  - preliminary signup to Issue Tracker
  - update Issue Tracker when starting coding
  - link to persistent Google+ Hangout