Scope extensions explainer: Add non-goals to extend other origin-keyed things

[mgiuca]

https://github.com/WICG/manifest-incubations/blob/gh-pages/scope_extensions-explainer.md

We should add a "non-goals" section which explains that scope extensions will not be targeting extending other parts of the web platform, besides the Manifest-centric concept of "scope", to other origins. For example (but not limited to):

• Same-origin policy checks.
• Storage keyed at the origin level.
• Permissions granted to specific origins.

[mgiuca]

@alancutter (trying to assign to you but you're not a member of the project; just invited).

[mgiuca]

@kenchris I'm not sure exactly what you're saying here (it doesn't seem very relevant to the non-goals section I'm requesting on this issue).