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

third_party/OWNERS

 # Owner approval for 3rd party is only required for
-# adding new libraries. For changes to existing code
-# simply TBR= one of people below.
+# adding new libraries. Be sure to follow the instructions for adding
+# new third party libraries.
+# http://www.chromium.org/developers/adding-3rd-party-libraries
+#
+# For changes to existing code simply TBR= one of people below.
 
 brettw@chromium.org
 cpu@chromium.org