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Add a pull request template
Add a pull request template

Summary:
Most libdef PRs do not include docs. Hopefully this will fix that problem.
Docs PRs are more likely to get quick attention if we know it's a doc pr without opening it, so hopefully this fixes that problem too.

Reviewed By: nmote

Differential Revision: D13864512

fbshipit-source-id: dfd72def1c448363cd518ab4788fb23cd204c818
