docs: Add instructions for obtaining a Gerrit cookie

Change-Id: If063079025a9628eaf5e50b8c990c75e7d334bed
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48322
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Keir Mierle <keir@google.com>

docs/contributing.rst

diff --git a/docs/contributing.rst b/docs/contributing.rst
index 92fe3c7..5a0defa 100644
--- a/docs/contributing.rst
+++ b/docs/contributing.rst
@@ -23,6 +23,7 @@
   - 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)
+  - 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
   - Install the Pigweed presubmit check hook with ``pw presubmit --install``