Document the requirement that commits have a public email address (#109318)

See
https://discourse.llvm.org/t/hidden-emails-on-github-should-we-do-something-about-it/74223
for details about why this is important to the community.

Note, we currently have soft enforcement for this requirement in the
form of a bot which posts comments letting patch authors know their
email is private, so we're already setting expectations in practice;
this PR is documenting those expectations for clarity.

Author: AaronBallman

llvm/docs/DeveloperPolicy.rst

@@ -105,6 +105,13 @@ When submitting patches, please do not add confidentiality or non-disclosure
 notices to the patches themselves.  These notices conflict with the LLVM
 licensing terms and may result in your contribution being excluded.
 
+The LLVM project uses email to communicate to contributors outside of the
+GitHub platform about their past contributions. Primarily, our buildbot
+infrastructure uses emails to contact contributors about build and test
+failures. Therefore, the LLVM community requires contributors to have a public
+email address associated with their GitHub commits, so please ensure that "Keep
+my email addresses private" is disabled in your
+`account settings <https://github.com/settings/emails>`_.
 
 .. _code review: