Make it explicit that creating a pull request is something that can't be done on the command line

[MikeyBoy1969]

In the Git command-line tutorial, there is an instruction to create a pull request:

http://dont-be-afraid-to-commit.readthedocs.org/en/latest/git/commandlinegit.html#send-me-a-pull-request

It would be helpful to make it explicit that this is not something that is done on the command-line, but via the GitHub website.