Begin fleshing out README with info from emberjs/rfcs#415.

README.md

@@ -1,7 +1,8 @@
 @ember/render-modifiers
 ==============================================================================
 
-[Short description of the addon.]
+Provides element modifiers that can be used to hook into specific portions of
+the rendering lifecycle.
 
 
 Compatibility
@@ -19,11 +20,201 @@ ember install @ember/render-modifiers
 ```
 
 
-Usage
+Usage Examples
 ------------------------------------------------------------------------------
 
-[Longer description of how to use the addon in apps.]
+### Example: Scrolling an element to a position
 
+This sets the scroll position of an element, and updates it whenever the scroll
+position changes.
+
+Before:
+
+```hbs
+{{yield}}
+```
+```js
+export default Component.extend({
+  classNames: ['scroll-container'],
+
+  didRender() {
+    this.element.scrollTop = this.scrollPosition;
+  }
+});
+```
+
+After:
+
+```hbs
+<div
+  {{did-insert this.setScrollPosition @scrollPosition}}
+  {{did-update this.setScrollPosition @scrollPosition}}
+
+  class="scroll-container"
+>
+  {{yield}}
+</div>
+```
+```js
+export default class Component.extend({
+  setScrollPosition(element, scrollPosition) {
+    element.scrollTop = scrollPosition;
+  }
+})
+```
+
+#### Example: Adding a class to an element after render for CSS animations
+
+This adds a CSS class to an alert element in a conditional whenever it renders
+to fade it in, which is a bit of an extra hoop. For CSS transitions to work, we
+need to append the element _without_ the class, then add the class after it has
+been appended.
+
+Before:
+
+```hbs
+{{#if shouldShow}}
+  <div class="alert">
+    {{yield}}
+  </div>
+{{/if}}
+```
+```js
+export default Component.extend({
+  didRender() {
+    let alert = this.element.querySelector('.alert');
+
+    if (alert) {
+      alert.classList.add('fade-in');
+    }
+  }
+});
+```
+
+After:
+
+```hbs
+{{#if shouldShow}}
+  <div {{did-insert this.fadeIn}} class="alert">
+    {{yield}}
+  </div>
+{{/if}}
+```
+```js
+export default Component.extend({
+  fadeIn(element) {
+    element.classList.add('fade-in');
+  }
+});
+```
+
+#### Example: Resizing text area
+
+One key thing to know about `{{did-update}}` is it will not rerun whenever the
+_contents_ or _attributes_ on the element change. For instance, `{{did-update}}`
+will _not_ rerun when `@type` changes here:
+
+```hbs
+<div {{did-update this.setupType}} class="{{@type}}"></div>
+```
+
+If `{{did-update}}` should rerun whenever a value changes, the value should be
+passed as a parameter to the modifier. For instance, a textarea which wants to
+resize itself to fit text whenever the text is modified could be setup like
+this:
+
+```hbs
+<textarea {{did-update this.resizeArea @text}}>
+  {{@text}}
+</textarea>
+```
+```js
+export default Component.extend({
+  resizeArea(element) {
+    element.css.height = `${element.scrollHeight}px`;
+  }
+});
+```
+
+#### Example: `ember-composability-tools` style rendering
+
+This is the type of rendering done by libraries like `ember-leaflet`, which use
+components to control the _rendering_ of the library, but without any templates
+themselves. The underlying library for this is [here](https://github.com/miguelcobain/ember-composability-tools).
+This is a simplified example of how you could accomplish this with Glimmer
+components and element modifiers.
+
+Node component:
+
+```js
+// components/node.js
+export default Component.extend({
+  init() {
+    super(...arguments);
+    this.children = new Set();
+
+    this.parent.registerChild(this);
+  }
+
+  willDestroy() {
+    super(...arguments);
+
+    this.parent.unregisterChild(this);
+  }
+
+  registerChild(child) {
+    this.children.add(child);
+  }
+
+  unregisterChild(child) {
+    this.children.delete(child);
+  }
+
+  didInsertNode(element) {
+    // library setup code goes here
+
+    this.children.forEach(c => c.didInsertNode(element));
+  }
+
+  willDestroyNode(element) {
+    // library teardown code goes here
+
+    this.children.forEach(c => c.willDestroyNode(element));
+  }
+}
+```
+```hbs
+<!-- components/node.hbs -->
+{{yield (component "node" parent=this)}}
+```
+
+Root component:
+
+```js
+// components/root.js
+import NodeComponent from './node.js';
+
+export default NodeComponent.extend();
+```
+```hbs
+<!-- components/root.hbs -->
+<div
+  {{did-insert (action this.didInsertNode)}}
+  {{will-destroy (action this.willDestroyNode)}}
+>
+  {{yield (component "node" parent=this)}}
+</div>
+```
+
+Usage:
+
+```hbs
+<Root as |node|>
+  <node as |node|>
+    <node />
+  </node>
+</Root>
+```
 
 License
 ------------------------------------------------------------------------------