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>
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