doc: document resolution config option and set it to 720 by default

config.js

@@ -80,5 +80,7 @@ var config = { // eslint-disable-line no-unused-vars
     // Suspending video might cause problems with audio playback. Disabling until these are fixed.
     disableSuspendVideo: true,
     // disables or enables RTX (RFC 4588).
-    disableRtx: true
+    disableRtx: true,
+    // Sets the preferred resolution (height) for local video. Defaults to 360.
+    resolution: 720
 };