Add instructions on using UMD build to Installation page

[samselikoff]

Someone on HN asked + we do already publish a UMD build. Just need to document it.

https://unpkg.com/browse/miragejs@0.1.33/dist/

[samselikoff]

I think just documenting a <script> tag pointing to unpkg would work, and then explain what the global is

[samselikoff]

Example from React guides

<script src="https://unpkg.com/react@16/umd/react.development.js" crossorigin></script>

Ours is

<script src="https://unpkg.com/miragejs/dist/mirage-umd.js" crossorigin></script>

[samselikoff]

Blocked by miragejs/miragejs#330