More clearly indicate status/relationship to WICG polyfill

[patrickhlauke]

When doing a search for "inert polyfill" results usually point to both this repo and the https://github.com/WICG/inert one, causing some confusion for developers.

while there is a note in https://github.com/GoogleChrome/inert-polyfill#limitations it would be good if the readme started more directly with some statement about the status of this polyfill, its relationship with the WICG one, and suggestion to developers on which one they should use/look at.