docs: New contributors should sign in to Gerrit

If this step is skipped, then trying to push a CL will yield a
PERMISSION_DENIED_BY_GERRIT_ACL error saying that "\'git push\'
requires a Gerrit user account".

Change-Id: I4d92e2a06f7b46ab6cb7be76a22c9be28a21d1a6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/76922
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Keir Mierle <keir@google.com>
diff --git a/docs/contributing.rst b/docs/contributing.rst
index 5385d4b..77a4bf1 100644
--- a/docs/contributing.rst
+++ b/docs/contributing.rst
@@ -23,6 +23,8 @@
   - Verify that your Git user email (git config user.email) is either Google
     Account email or an Alternate email for the Google account used to sign
     the CLA (Manage Google account → Personal Info → email)
+  - Sign in to `Gerrit <https://pigweed-review.googlesource.com/>`_ to create
+    an account using the same Google account you used above.
   - Obtain a login cookie from Gerrit's `new-password <https://pigweed-review.googlesource.com/new-password>`_ page
   - Install the Gerrit commit hook to automatically add a ``Change-Id: ...``
     line to your commit