[llvm] [llvm][docs] Extend docs on GitHub's "squash and merge" (PR #129497)

[ofri frishman via llvm-commits]

================
@@ -15,6 +15,12 @@ The LLVM Project uses `GitHub <https://github.com/>`_ for
 This page describes how the LLVM Project users and developers can
 participate in the project using GitHub.
 
+Before your first PR
+====================
+
+Please ensure that you have set a valid email address in your GitHub account,
+see :ref:`github-email-address`.
----------------
ofri-frishman wrote:

Yeah, seems good. I didn't manage to find what the link pointed to, so didn't see the relevant text in the link. Thanks.

https://github.com/llvm/llvm-project/pull/129497