Add contains to includes RFC

Author: alexspeller

text/0000-contains-to-includes.md

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+- Start Date: 2016-04-16
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# Summary
+
+[`contains`](http://emberjs.com/api/classes/Ember.Array.html#method_contains) is
+implemented on `Ember.Array`, but [contains was renamed to includes in 2014]
+(https://github.com/tc39/Array.prototype.includes/commit/4b6b9534582cb7991daea3980c26a34af0e76c6c)
+- this proposal is for `contains` to be deprecated in favour of an `includes`
+method on `Ember.Array`
+
+# Motivation
+
+Motivation is to stay in line with web standards
+
+# Detailed design
+
+* Rename `contains` method to `includes`
+* Alias `contains` with deprecation warning, deprecate in line with standard
+deprecation process
+
+# How We Teach This
+
+Update any references in docs and guides to `includes`
+
+# Drawbacks
+
+* May break existing apps
+* [Was considered before but was too early](https://github.com/emberjs/ember.js/issues/5670#issuecomment-64084814)
+
+# Alternatives
+
+Keep current methods
+
+# Unresolved questions
+
+None