Contributing Code: Link to GitHub's guides on pull requests.

Every once in a while, someone wants to contribute patches but does not
know how to get started with GitHub's workflow.

Provide a few links to GitHub's "Fork a repository" and "Using pull
requests" guides on the list of steps for submitting a patch.
Merge pull request #157 from rakuco/contribute-code-link-to-github

Contributing Code: Link to GitHub's guides on pull requests.