Update readme to reflect callbacks

It's very likely, these commands will need to happen serially, therefore, it's important to ensure that gulp knows to wait for task completion before proceeding. This clarifies the docs to show that commands accept a callback or are part of a stream.
Only some examples need to show callbacks

Per https://github.com/stevelacy/gulp-git/pull/10#issuecomment-36665234
Merge pull request #10 from joeybaker/patch-1

Update readme to reflect callbacks