How to write a PR tutorial in CONTRIBUTING.md

[stephenroller]

We often get people who are interested in contributing, but don't know how to create a PR, especially if they've followed the steps of cloning the parlai repo directly.

Please add a runbook of instructions on how to create a PR in CONTRIBUTING.md.

The instructions should have two possible paths:

• Creating a github fork and working there (the "standard" path). This can heavily link to the GitHub documents
• "I've already cloned ParlAI and committed my changes. How do I move to my fork?"

Additionally, add a link to CONTRIBUTING.md to an appropriate place in the parlai docs.

[github-actions]

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