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docs: add chromium sheriff guide

This doc outlines how I practice sheriffing and how I encourage
others to do so, along with links and advice for how to deal
with specific sheriffing tasks.

Bug: None
Change-Id: I2eaa3217ea3c38f1cee977a03060aca281fb401b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632569
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664281}