Add notes about other webkitpy thirdparty checked-in libraries.

This provides a bit more of a paper trail for the libraries we
already had checked in, and makes things a bit more consistent
with how we track stuff elsewhere in chromium.

(This patch depends on https://codereview.chromium.org/19481002/ ).

R=ojan@chromium.org
BUG=230706

Review URL: https://codereview.chromium.org/19484002

git-svn-id: svn://svn.chromium.org/blink/trunk@154443 bbb929c8-8fbe-4397-9dbb-9b2b20218538