Flesh out README.

README.md

@@ -1,14 +1,15 @@
 ember-modifier-manager-polyfill
 ==============================================================================
 
-[Short description of the addon.]
+This addon provides a polyfill for implementing element modifier managers as
+described in [emberjs/rfcs#373](https://emberjs.github.io/rfcs/0373-Element-Modifier-Managers.html).
 
 
 Compatibility
 ------------------------------------------------------------------------------
 
-* Ember.js v2.18 or above
-* Ember CLI v2.13 or above
+* Completely inert when running `ember-source` 3.8 or higher
+* Tested against `ember-source` v2.12, v2.16, v2.18, v3.4, v3.5, v3.6, v3.7 in CI
 
 
 Installation
@@ -22,7 +23,116 @@ ember install ember-modifier-manager-polyfill
 Usage
 ------------------------------------------------------------------------------
 
-[Longer description of how to use the addon in apps.]
+At this point the best documentation is likely still the [the RFC
+itself](https://emberjs.github.io/rfcs/0373-Element-Modifier-Managers.html),
+but here are a few simple examples (shamelessly stolen from another modifier
+RFC [emberjs/rfcs#353](https://github.com/emberjs/rfcs/pull/353)):
+
+### Performance Marking
+
+```hbs
+<section id="about-us" {{performance 'mark' 'about-page'}}>
+  <h1>About Us</h1>
+  {{!-- snip --}}
+</section>
+```
+
+```js
+// app/modifiers/performance.js
+import Ember from 'ember';
+
+export default Ember._setModifierManager(
+  () => ({
+    createModifier() {},
+
+    installModifier(_instance, _element, args) {
+      let [type, name] = args.positional;
+
+      performance[type](name);
+    },
+
+    updateModifier() {},
+    destroyModifier() {},
+  }),
+  class PerformanceModifier {}
+);
+```
+
+### jQuery Widget
+```hbs
+<input type="date" {{datepicker changeMonth=true changeYear=true}} />
+```
+
+```js
+// app/modifiers/datepicker.js
+import Ember from 'ember';
+import $ from 'jquery';
+
+export default Ember._setModifierManager(
+  () => ({
+    createModifier() {
+      return { element: null };
+    },
+
+    installModifier(state, element, args) {
+      let options = Object.assign({ minDate: 20, maxDate: '+1M +10D' }, args.named);
+
+      // setup state bucket for use in destroyModifier
+      state.element = element;
+      state.options = options;
+
+      $(this.element).datepicker(options);
+    },
+
+    updateModifier() {},
+    destroyModifier(state) {
+      $(state.element).datepicker(state.options);
+    },
+  }),
+  class DatepickerModifier {}
+);
+```
+
+### Page View Tracking
+```hbs
+<section {{track-impression eventCategory="Post"}}>
+  <header>Chad liked a post</header>
+  <img src="cat.jpg">
+  {{!-- Snip --}}
+<section>
+```
+
+```js
+// app/modifiers/track-impression.js
+import Ember from 'ember';
+
+export default Ember._setModifierManager(
+  owner => ({
+    createModifier() {
+      return { element: null, observer: null };
+    },
+
+    installModifier(state, element, args) {
+      let ga = owner.lookup('service:ga');
+      let { eventCategory } = args.named;
+
+      let interSectionObserver = new IntersectionObserver(entries => {
+        entries.forEach(entry => ga.send('event', 'impression', eventCategory));
+      });
+
+      // setup state bucket for use in destroyModifier
+      state.element = element;
+      state.observer = interSectionObserver;
+    },
+
+    updateModifier() {},
+    destroyModifier(state) {
+     state.observer.unobserve(state.element);
+    },
+  }),
+  class TrackImpressionModifier {}
+);
+```
 
 
 License