Update README

[tedbow]

I am trying to catch up on the JS Admin core initiative.
I found this repo via: https://www.drupal.org/about/strategic-initiatives/admin-ui-js
which says:

To help with the refreshed admin theme, head to
but then this repo doesn't give any info on how it relates to other issues.

[ckrina]

Things to update on the Readme:

• Installation instructions explaining how to have both default core Seven them and this one a a custom one overriding the old
• Define the role in core this initiative has.