Customtype Warnings and issues update (#479)

* added issue link to customtype warnings and issue list

* added issue #478 to the customtype warning and issue list

custom_types.md

@@ -67,3 +67,5 @@ Issues with customtype include:
   * <a href="https://github.com/gogo/protobuf/issues/125">Using a proto message as a customtype is not allowed.</a>
   * <a href="https://github.com/gogo/protobuf/issues/200">cusomtype of type map can not UnmarshalText</a>
   * <a href="https://github.com/gogo/protobuf/issues/201">customtype of type struct cannot jsonpb unmarshal</a>
+  * <a href="https://github.com/gogo/protobuf/issues/477">Customtype field does not get a generated 'getter' method</a>
+  * <a href="https://github.com/gogo/protobuf/issues/478">Repeated customtype fields generate slices without pointer to the custom type </a>