Mention the 'adding third party libraries' URL in third_party/OWNERS.

NOTRY=true

Review URL: https://codereview.chromium.org/127203003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243472 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243472