docs: Document workaround for arcanist failures

Reviewed By: smeenai

Differential Revision: https://reviews.llvm.org/D110976

llvm/docs/GettingStarted.rst

@@ -467,8 +467,16 @@ corresponding to the review you wish to send, up-to-date with the upstream
 can start `a Phabricator review <Phabricator.html>`_ (or use ``git show`` or
 ``git format-patch`` to output the diff, and attach it to an email message).
 
-However, using the "Arcanist" tool is often easier. After `installing
-arcanist`_, you can upload the latest commit using:
+However, using the "Arcanist" tool is often easier. After `installing arcanist`_, you
+will also need to apply a fix to your arcanist repo in order to submit a patch:
+
+.. code-block:: console
+
+  % cd arcanist
+  % git fetch https://github.com/rashkov/arcanist update_cacerts
+  % git cherry-pick e3659d43d8911e91739f3b0c5935598bceb859aa
+
+Once this is all done, you can upload the latest commit using:
 
 .. code-block:: console