Update README with more developer info
Update README with more developer info

- add: Quick Start guide to get new developers up-and-running fast
- add: Contributing to refer new contributors to the relevant
       documentation/guidelines

bjmllr/habitat_humanity,bjmllr/habitat_humanity,rubyforgood/habitat_humanity,rubyforgood/habitat_humanity,rubyforgood/habitat_humanity,bjmllr/habitat_humanity