I have created a PR for RtpEncodingParameters.maxBitrate

Here's the PR:

https://github.com/w3c/webrtc-pc/pull/254

Here's the diff:

+
+          <dt>unsigned long maxBitrate</dt>
+          <dd>
+            <p>
+              Indicates the maximum bitrate that can used to send this
+              encoding.
+            </p>
+          </dd>


Question for the WG:  do we have consensus to add this?   I'm in favor of
it :).