Adapt Arthur's lit-html README for lit-element (#623)
Adapt Arthur's lit-html README for lit-element (#623)

* Adapt Arthur's lit-html README for lit-element

* lit-html -> lit-element

Co-Authored-By: katejeffreys <cd67cce45b98f184e14455567299268a7e8d154d@google.com>

lit/lit-element,Polymer/lit-element,lit/lit-element,lit/lit-element,Polymer/lit-element,Polymer/lit-element