Add informative section on legacy constructs (#506)

index.bs

@@ -13396,6 +13396,29 @@ Extensions to any other aspect of the IDL language are
 strongly discouraged.
 
 
+<h2 id="legacy-constructs">Legacy constructs</h4>
+
+<i>This section is informative.</i>
+
+Legacy WebIDL constructs exist only so that
+legacy Web platform features can be specified.
+They are generally prefixed with the "<code>Legacy</code>" string.
+It is strongly discouraged to use legacy WebIDL constructs in specifications
+unless required to specify the behavior of legacy Web platform features,
+or for consistency with such features.
+Editors who wish to use legacy WebIDL constructs are strongly advised to discuss this
+by <a href="https://github.com/heycam/webidl/issues/new?title=Intent%20to%20use%20a%20legacy%20WebIDL%20construct">filing an issue</a>
+before proceeding.
+
+Marking a construct as legacy does not, in itself,
+imply that it is about to be removed from this specification.
+It does suggest however, that it is a good candidate
+for future removal from this specification,
+whenever various heuristics indicate that
+the Web platform features it helps specify can be removed altogether
+or can be modified to rely on non-legacy WebIDL constructs instead.
+
+
 <h2 id="referencing">Referencing this specification</h2>
 
 <i>This section is informative.</i>