Added core/css/properties/README.md.

Added core/css/properties/README.md, detailing the purpose and status of
CSS property APIs as well as the steps involved in adding a new
property API.

BUG=545324

Review-Url: https://codereview.chromium.org/2746793002
Cr-Commit-Position: refs/heads/master@{#457305}
1 file changed