Update thumbnails of github interface
Add labels for cross-referencing
Move virtualenv information to a separate document
Move new contributing docs into workflow folder
Extensive rewrite of workflow for developers, with some details moved to separate files

The rewrite focus moron the needs of new contributors, less on the needs of experienced developers.
Note that virtualenvwrapper does not work in all shells

…and a few formatting changes
Add recommendation to use virtual environments
Simplify list of ways to contribute
Make developer documentation a top-level heading separate from contributing
Incorporate clarifications to additional git topics suggested by @mhkv
Revise index page so that contributing more closely mirrors other sections

Responds to comments by both @taldcroft and @mhkv
Modify development workflow title/intro to reflect changes on index page
Add advice about where to start in making first contribution
Clean up rst/whitespace issues
Add worked example, supporting file and links to the example
Fix link to terminal-cast page
Fix typos and add shortcut to contribution workflow for experienced developers
Explicitly mention direct email for providing astropy feedback
Another typo fix
Add another reference to virtual environment documentation