[Docs] Instructions for new contributors to find reviewers (llvm#71936)

Right now, the documentation does not clarify what new contributors
should do to find potential reviewers, since new contributors don't have
write access and thus cannot select them as explained in the docs.

This patch extends the guidelines for new people, as discussed in
https://discourse.llvm.org/t/how-to-find-reviewers/74803

Author: pabloantoniom

llvm/docs/Contributing.rst

@@ -98,7 +98,10 @@ To make sure the right people see your patch, please select suitable reviewers
 and add them to your patch when requesting a review. Suitable reviewers are the
 code owner (see CODE_OWNERS.txt) and other people doing work in the area your
 patch touches. Github will normally suggest some reviewers based on rules or
-people that have worked on the code before.
+people that have worked on the code before. If you are a new contributor, you
+will not be able to select reviewers in such a way, in which case you can still
+get the attention of potential reviewers by CC'ing them in a comment -- just
+@name them.
 
 A reviewer may request changes or ask questions during the review. If you are
 uncertain on how to provide test cases, documentation, etc., feel free to ask