Merge to XFA: [Docs] Add wiki content to Markdown docs

(cherry picked from commit 61327fe3c46b73141b4c885a98e97cd5d1412aba)
Original Review URL: https://codereview.chromium.org/1356323002 .

TBR=andybons@chromium.org

Review URL: https://codereview.chromium.org/1359813003 .
3 files changed