Mojo JS Bindings User's Guide

An overview of the JS bindings from a user's point of view.

Much of this material has appeared in CL descriptions, even email,
where no one is likely to find it. Now it's in the the README,
along with the JS examples.

This isn't complete and some of the material will definitely change.
It's a start.

BUG=
R=eseidel@google.com

Review URL: https://codereview.chromium.org/776133003