explicitly state that this policy is limited to TPA in scope

This also introduces a link to the core membership policy which we
refer to elsewhere in the document.

policy/tpa-rfc-7-root.md

@@ -64,6 +64,13 @@ on the servers they manage (and only those). It also tries to define a
 security policy for access tokens, as well as admission and revocation
 policies.
 
+## Scope
+
+This policy complements the [Tor Core Membership policy][] but
+concerns only membership to the TPA team and access to servers.
+
+[Tor Core Membership policy]: https://gitweb.torproject.org/community/policies.git/plain/membership.txt
+
 ## Access levels
 
 Members of TPA SHOULD have all access levels defined above.