README updates - Githubissues

Made updates to the README for clarity after some confusion from an applicant:

Added specific proejct setup instructions for working in c9 and locally. Moved these out from under "Setup" which previously said it could be skipped.
Added instructions for running specs and explainer on Gemfile/bundler/bundle install -- these don't seem to be covered anywhere else.

appacademy / test-first-ruby