Merge to XFA: Add some community and testing info to README.md.

This CL adds some more context to the README on the various test suites and
community mailing lists which are available.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1412453004 .

(cherry picked from commit 51cf5bc00dfa3033523355e3e74f3a47f7e46682)

This merge had conflicts.

Review URL: https://codereview.chromium.org/1413523002 .
1 file changed