Add requirments section for de-duping between services.

proposals/2140-terms-of-service-2.md

@@ -20,6 +20,11 @@ Requirements for this proposal are:
  * A user should only have to agree to each version of each document once for
    their Matrix ID, ie. having agreed to a set of terms in one client, they
    should not have to agree to them again when using a different client.
+ * Documents should be de-duplicated between services. If two or more services
+   are hosted by the same organisation, the organistation should have the
+   option to give their users a single document that encompasses both services
+   (bearing in mind that the user must be able to opt-out of components of a
+   service whilst still being able to use the service without that component).
 
 ## Proposal