Deprecate {{partial}} (#456)

Deprecate `{{partial}}`

content/ember/v3/partial.md

@@ -0,0 +1,40 @@
+---
+id: ember.partial
+title: Deprecate `{{partial}}`
+until: '4.0.0'
+since: '3.15'
+---
+
+We are deprecating usage of `{{partial}}` in accordance with [RFC #449](https://github.com/emberjs/rfcs/blob/master/text/0449-deprecate-partials.md).
+
+Partials should be migrated to components. For example, consider the following `quick-tip` partial:
+
+```hbs
+{{! app/templates/application.hbs }}
+{{#let (hash title="Don't use partials" body="Components are always better") as |tip|}}
+  {{partial "partials/quick-tip"}}
+{{/let}}
+```
+
+```hbs
+{{! app/templates/partials/quick-tip.hbs }}
+<h1>Tip: {{tip.title}}</h1>
+<p>{{tip.body}}</p>
+<button {{on "click" this.dismissTip}}>OK</button>
+```
+
+It can be converted to a component as follows:
+
+```hbs
+{{! app/templates/application.hbs }}
+{{#let (hash title="Don't use partials" body="Components are always better") as |tip|}}
+  <QuickTip @tip={{tip}} @onDismiss={{this.dismissTip}} />
+{{/let}}
+```
+
+```hbs
+{{! app/templates/components/quick-tip.hbs }}
+<h1>Tip: {{@tip.title}}</h1>
+<p>{{@tip.body}}</p>
+<button {{action @onDismiss}}>OK</button>
+```