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Move C++ Do's and Dont's file to Markdown

This doesn't change any content of the file. To create this patch, I:
1. Used a script to generate markdown from the HTML
2. Cleaned up differences by hand
3. Formatted and wrapped to 80 chars
4. Added 'DO' and 'DONT' in front of code blocks where the red/green
   colouring was lost (and the difference is important)

Change-Id: Ib16c404ce5cf8f4e0d434e35886037dafbcc63a5
Reviewed-on: https://chromium-review.googlesource.com/802843
Commit-Queue: Sasha Morrissey <sashab@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521941}