Explain how to load TileJSON from CDN

[rgov]

I tried loading the dist.js file into my document and that caused a big headache because this file apparently includes an old version of Leaflet. I guess it's just meant for running the examples, and I should have used index.js instead.

It would be helpful to show in the README how to load the library from a CDN.

<!-- load Leaflet first -->
<script src="https://unpkg.com/proj4@2.5.0/dist/proj4.js"></script>
<script src="https://unpkg.com/proj4leaflet@1.0.2/src/proj4leaflet.js"></script>
<script src="https://unpkg.com/leaflet-tilejson@1.0.0-rc4/index.js"></script>