docs(guide): Add a ModalDialog guide (#3961)

docs/guides/modal-dialog.md

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+# Using the Modal Dialog Component
+
+The `ModalDialog` component is part of Video.js core and provides a baked-in UI for full-player overlays.
+
+## Table of Contents
+
+* [Creating the ModalDialog](#creating-the-modaldialog)
+  * [Example Using createModal()](#example-using-createmodal)
+  * [Example Using the ModalDialog Constructor](#example-using-the-modaldialog-constructor)
+* [Styling Modals Independently](#styling-modals-independently)
+
+## Creating a ModalDialog
+
+Aside from the [built-in Video.js component-creation methods][creating-component], the player includes a `createModal()` helper method.
+
+We'll demonstrate both approaches in this document by creating a modal that opens when the player becomes paused and resumes playback when it is closed.
+
+### Example Using `createModal()`
+
+The `createModal()` method is intended for creating one-off modals that need to open for some temporary purpose. Therefore, they open themselves immediately upon creation and, by default, dispose themselves immediately upon closing.
+
+```js
+var player = videojs('my-player');
+
+player.on('pause', function() {
+
+  // Modals are temporary by default. They dispose themselves when they are
+  // closed; so, we can create a new one each time the player is paused and
+  // not worry about leaving extra nodes hanging around.
+  var modal = player.createModal('This is a modal!');
+
+  // When the modal closes, resume playback.
+  modal.on('modalclose', function() {
+    player.play();
+  });
+});
+```
+
+The `createModal()` method also takes a second argument - an object containing options for the modal. Refer to [the API documentation][api-doc] for a full set of options.
+
+### Example Using the `ModalDialog` Constructor
+
+Unlike when using `createModal()`, a modal created with any of the [common component creation methods][creating-component] _does not_ open by default. This makes this approach better suited to modals that are expected to live in the DOM indefinitely.
+
+```js
+var player = videojs('my-player');
+var ModalDialog = videojs.getComponent('ModalDialog');
+
+var modal = new ModalDialog(player, {
+
+  // We don't want this modal to go away when it closes.
+  temporary: false
+});
+
+player.addChild(modal);
+
+player.on('pause', function() {
+  modal.open();
+});
+
+player.on('play', function() {
+  modal.close();
+});
+```
+
+Both of these examples are equivalent when it comes to the user's experience. Implementors should use whichever better suits their use-case.
+
+## Styling Modals Independently
+
+A common need for modals is to style them independently from one another. The recommended approach for this is to add a custom class to your modal and target that using CSS:
+
+```js
+modal.addClass('vjs-my-fancy-modal');
+```
+
+[api-doc]: http://docs.videojs.com/docs/api/modal-dialog.html
+
+[creating-component]: ./components.md#creating-a-component