Add section to CONTRIBUTING.md explaining developer docs (#63228)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63228
It is a quick summary and links to a page on the Developer Wiki that has
more detail.
Test Plan: Imported from OSS
Reviewed By: SplitInfinity
Differential Revision: D30347109
Pulled By: zou3519
fbshipit-source-id: a6242986d275e5279ca3f61ade2294a132d268c4
