PCScan: Add documentation

Adds a markdown documentation file that generally describes heap
scanning algorithms and more specifically PCScan

Bug: 11297512
Change-Id: Ic863be301dca23bab38f6026afd549bea1c30a0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2775561


Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864938}