[firestore-shorten-urls-dynamic-links] Note that created links won't show up in Firebase console

[jhuleatt]

We should add a note to POSTINSTALL.md to explain that dynamic links created by this extension won't show up in the console.

Expected Behavior

Dynamic links created by the extension would show up in the FDL console

Actual Behavior

They don't, but it turns out this is expected per the Firebase docs

Short Dynamic Links created with the API and client (Android/iOS) SDK do not show up in the Firebase console.

[jhuleatt]

FYI @kevinthecheung

[cabljac]

Note: with dynamic links now deprecated, need to discuss this extension with the team