Rename borderless to unframed and update explainer with url_patterns (#119)

'unframed' display mode, allow `display_override` dict entries, `display` & `url_patterns`.

Apps can then declare which URLs should be unframed via `url_patterns`.
This enables mixed scenarios where some windows are unframed and some
are normal.

Author: edman

README.md

@@ -16,6 +16,7 @@ There are a number of [Web App Manifest](https://www.w3.org/TR/appmanifest/) fea
 - [Tabbed mode](https://github.com/w3c/manifest/issues/737)
 - [Protocol handlers](https://github.com/w3c/manifest/issues/846)
 - [Declarative link capturing](https://github.com/WICG/sw-launch/blob/master/declarative_link_capturing.md)
+- [Unframed mode](https://github.com/WICG/manifest-incubations/issues/118)
 
 Rather than creating a separate WICG repo for each of these (given that there are often interdependencies between them), we are creating a "manifest-incubations" spec with all of them in a single document. That means readers only need to keep track of two documents, instead of ~10.