Add some clarification to email check message

Author: asl

.github/workflows/email-check.yaml

@@ -40,4 +40,5 @@ jobs:
               repo: context.repo.repo,
               body: `⚠️ We detected that you are using a GitHub private e-mail address to contribute to the repo.
               Please turn off [Keep my email addresses private](https://github.com/settings/emails) setting in your account.
+              See [LLVM Discourse](https://discourse.llvm.org/t/hidden-emails-on-github-should-we-do-something-about-it) for more information.
             `})