Update README with info regarding non-PR refspec

Using the default `refspec` prohibits the project from automatically building non-PR branches like `origin/master`. Prepending the `refspec` with the heads and remote/origin allows the project to build pull requests as well as other branches like `origin/my-feature`. This should resolve #77.

Big thanks to https://github.com/janinko/ghprb/issues/77#issuecomment-40400379 :smile: