Updated the 'Installing Astropy in develop mode' section (use setup.py develop in the first instance, added a link to the instructions on how to run the tests, which required adding a label to install.rst), added some additional links to GitHub pages, fixed some typos, applied other minor edits for clarity / consistency
Merge pull request #1212 from ehogan/dev-workflow-doc-improvements

Updated the 'Installing Astropy in develop mode' section ...