[libc++][NFC] Add link to Discord channel from documentation

NOKEYCHECK=True
GitOrigin-RevId: b034593c8719e42a56cf4e6cb2ffb762783b0d7f
diff --git a/docs/Contributing.rst b/docs/Contributing.rst
index 9ff7d6c..8dc1856 100644
--- a/docs/Contributing.rst
+++ b/docs/Contributing.rst
@@ -12,6 +12,9 @@
 for submitting a code review from the command-line using ``arc``, since we have some
 automation (e.g. CI) that depends on the review being submitted that way.
 
+If you plan on contributing to libc++, it can be useful to join the ``#libcxx`` channel
+on `LLVM's Discord server <https://discord.gg/jzUbyP26tQ>`__.
+
 Looking for pre-existing reviews
 ================================