Add contains to includes RFC

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
+
+First, implement `includes` polyfill in compliance with `includes` spec. Polyfill
+sample from MDN is:
+
+```js
+if (!Array.prototype.includes) {
+  Array.prototype.includes = function(searchElement /*, fromIndex*/ ) {
+    'use strict';
+    var O = Object(this);
+    var len = parseInt(O.length) || 0;
+    if (len === 0) {
+      return false;
+    }
+    var n = parseInt(arguments[1]) || 0;
+    var k;
+    if (n >= 0) {
+      k = n;
+    } else {
+      k = len + n;
+      if (k < 0) {k = 0;}
+    }
+    var currentElement;
+    while (k < len) {
+      currentElement = O[k];
+      if (searchElement === currentElement ||
+         (searchElement !== searchElement && currentElement !== currentElement)) { // NaN !== NaN
+        return true;
+      }
+      k++;
+    }
+    return false;
+  };
+}
+```
+
+Then, alias `contains` to `includes` with deprecation warning, deprecate in line with standard
+deprecation process. I don't believe that adding the additional parameter will
+have any affect on existing usage of `contains`.
+
+# How We Teach This
+
+* Update any references in docs and guides to `includes`
+* Write a deprecation guide, mentioning any edge cases where the new `includes` behaves differently to `contains`, and giving migration examples
+* Indicate in api docs that this is a polyfill
+
+# 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