Updated config/database.yml.example and CONTRIBUTING.md

config/database.yml.example now includes a username (system username) and password ("rubygemsrock") for the development and testing environments

CONTRIBUTING.md fills in the missing details on how to properly set up the development environment so that the "rake db:create:all db:drop:all db:setup db:test:prepare --trace" command works without errors.

Please note that these are the ONLY two files that have been changed.