Re-do readme as a high-level overview and plan

Should serve a bit better as the project's public face.

(Please don't use the merge button; if you give me a LGTM I'll merge it in a straight-line history.)

Moved most of the original readme to the wiki: https://github.com/dglazkov/html-as-custom-elements/wiki/Dimitri%27s-original-README-thoughts