Introduce recommendations for external specification modules

index.bs

@@ -474,8 +474,16 @@ An implementation may define <dfn export>extension modules</dfn>. These must hav
 [=module name=] that contains a single colon "<code>:</code>" character. The
 part before the colon is the prefix; this is typically the same for all
 [=extension modules=] specific to a given implementation and should be unique for a
-given implementation. Such modules extend the [=local end definition=] and [=remote end definition=]
-providing additional groups as choices for the defined [=commands=] and [=events=].
+given implementation.
+Such modules extend the [=local end definition=] and [=remote end definition=]
+providing additional groups as choices for the defined [=commands=], [=events=] and types.
+
+External specifications may define their own modules that extend the protocol.
+The recommendation is that such modules should have a [=module name=] that does
+not contain a colon "<code>:</code>" character, and that it is preferred
+to add new modules rather than extending existing ones.
+Such modules may extend the [=local end definition=] and [=remote end definition=] providing
+additional groups as choices for the defined [=commands=], [=events=] and types.
 
 ## Commands ## {#commands}