Merge pull request #19 from webscreens/issue-14-extensibility

Addresses Issue #4: Adds a note about extensibility.

requirements.md

@@ -151,3 +151,10 @@ TBD
 5. The controller initiating presentation should communicate its preferred
    locale to the display, so it can render the presentation content in that
    locale.
+
+### <a name="req-nf4-extend"></a>Extensibility
+
+1. It should be possible to extend the application level protocol (above the
+   discovery and transport levels) with optional features not defined explicitly
+   by the specification, to facilitate experimentation and enhancement of the
+   base protocols.