Add CONTRIBUTING.md

[rachitnigam]

Add a file describing how to the contribute the repo. Mentioned by @YJDoc2 in a recent PR.

[anthonyabeo]

@rachitnigam I will like to work on this. Does the team have anything specific expectations for this document?

[rachitnigam]

Hey @anthonyabeo! This one is a little tricky because we need to document a bunch of internal policies. I would recommend working on something else instead.

[anthonyabeo]

noted. @rachitnigam