Update the attribution policy to use the 'Author' property of a git commit

Summary:
The older method of adding 'Patch by John Doe' is documented in the
`Attribution of Changes` section to support correct attribution of commits
that pre-date the adoption of git.

Reviewers: hfinkel, aaron.ballman, mehdi_amini

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72468