Add advice about read-only lists

[atanassov]

The added recommendation is about the types of static lists to be considered for API design.
Addresses #50

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[LeaVerou]

What happened to ReadOnlyArray? Was it folded into ObservableArray as a special case?

[cynthia]

@plinss and I discussed this in a breakout during the F2F. We think the ObservableArray recommendation is a bit strange - the observable properties are definitely not desirable by every case where an attribute is needed, and there are cases where read-only is desirable (e.g. in an attribute) - but this PR doesn't seem to cover that.

Also, @plinss mentioned he wasn't so sure about the return being restricted to only sequences. There are cases where other array types are more desirable, but this feels a bit restrictive.

[annevk]

@LeaVerou I'm not aware of a ReadOnlyArray proposal, but https://heycam.github.io/webidl/#idl-observable-array does indeed satisfy that use case (it's basically up to API designers to decide what kind of operations they permit).

@domenic was it considered to have default implementations of delete and set an indexed value so that the readonly case would be consistent across APIs? That does strike me as something that is missing, but I might also be missing something. 😊

[domenic]

The plan was to create a ReadonlyArray Web IDL type which has those defaults and uses the same underlying infrastructure as ObservableArray.

[annevk]

Ah okay, so I guess we're mainly waiting for a consumer of that. Thanks!

[atanassov]

Had another round of CR with @plinss and @cynthia and arrived with the final wordsmithing addressing all concerns for this PR. Thank you all for the time.