doc: clarify collaborators & ctc members relationships

inspired by this conversation
https://github.com/nodejs/node/pull/7183#discussion_r73786541

PR-URL: https://github.com/nodejs/node/pull/7996
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

README.md

@@ -319,8 +319,9 @@ more information about the governance of the Node.js project, see
 * [zkat](https://github.com/zkat) -
 **Kat Marchán** <kzm@sykosomatic.org>
 
-Collaborators & CTC members follow the [COLLABORATOR_GUIDE.md](./COLLABORATOR_GUIDE.md) in
-maintaining the Node.js project.
+Collaborators (which includes CTC members) follow the
+[COLLABORATOR_GUIDE.md](./COLLABORATOR_GUIDE.md) in maintaining the Node.js
+project.
 
 ### Release Team