Update README.markdown

Added a more convenient example of retrieving the GitHub password from the Mac OS keychain. Using "find-internet-password" instead of "find-generic-password" means that a user need not maintain the Keychain entry manually if they change their password, as it will be updated automatically by their browser when they next log into the GitHub website successfully.
Merge pull request #127 from cliffrowley/patch-1

Update README.markdown