Expand extensions modules recommendation

Bug: #587

index.bs

@@ -477,6 +477,11 @@ part before the colon is the prefix; this is typically the same for all
 given implementation. Such modules extend the [=local end definition=] and [=remote end definition=]
 providing additional groups as choices for the defined [=commands=] and [=events=].
 
+When defining [=extension modules=], implementers should consider the
+possibility of future standardization of the module. If the module is
+likely to be standardized in the future, implementers should use a
+[=module name=] that does not contain a colon "<code>:</code>" character.
+
 ## Commands ## {#commands}
 
 A <dfn export>command</dfn> is an asynchronous operation, requested by