Add once-for-observer RFC

text/0000-once-for-observer.md

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+- Start Date: 2018-12-28
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# Add `once` flag to observer
+
+## Summary
+
+> `addListener` has a flag called `once`. This flag could be useful for observers as well.
+
+## Motivation
+
+> Adding the flag `once` to observers would improve the consistency between `addListener` and `addObserver`.
+
+## Detailed design
+
+> The `once` flag of `addListener` tells to call the method only once. This RFC aims to add this flag to observers as well.
+
+## How we teach this
+
+> The best would be to follow the terminology used in listeners because these are already known by Ember developers.
+
+> Only the API should be rebuild after this RFC.
+
+> This feature does not need big publicity. This should be only mentioned in the release note.
+
+## Drawbacks
+
+> Observers are not very popular in EmberJS.
+
+> There are no really tradeoffs except that observers would get a bit publicity.
+
+## Alternatives
+
+> The addon `ember-observer-macros` can be used. It has `observerOnce`.
+
+## Unresolved questions