Document the new Event Filters feature

Author: brandonkelly

README.md

@@ -72,7 +72,7 @@ The Sender Class can be a subclass of the class that triggers the event. For exa
 
 See [Integrating with Task Automation Tools](#integrating-with-task-automation-tools) for examples on how to get a Webhook URL from various task automation tools.
 
-![Screenshot of the Edit Webhook page](./screenshot.png)
+![Screenshot of the Edit Webhook page](./images/edit-webhook.png)
 
 Webhooks can either send a GET request, or a POST request with a JSON body containing the following keys:
 
@@ -84,6 +84,54 @@ Webhooks can either send a GET request, or a POST request with a JSON body conta
 - `eventClass` – the class name of the event.
 - `event` – an object with keys representing any event class properties that aren’t declared by [yii\base\Event](https://www.yiiframework.com/doc/api/2.0/yii-base-event). (For example, if a [craft\events\ElementEvent](https://docs.craftcms.com/api/v3/craft-events-elementevent.html) is triggered, this will contain [element](https://docs.craftcms.com/api/v3/craft-events-elementevent.html#property-element) and [isNew](https://docs.craftcms.com/api/v3/craft-events-elementevent.html#property-isnew) keys.)
 
+#### Filtering Events
+
+Some events can have filters applied to them, which prevent webhooks from being executed under certain conditions.
+
+![Screenshot of the Event Filters setting](./images/event-filters.png)
+
+Only element class events and certain `craft\services\Elements` events have any filters out of the box, but modules and plugins can register additional filters using the `craft\webhooks\Plugin::EVENT_REGISTER_FILTER_TYPES` event.
+
+```php
+use yii\base\Event;
+use craft\webhooks\Plugin as Webhooks;
+use craft\events\RegisterComponentTypesEvent;
+
+Event::on(Webhooks::class, Webhooks::EVENT_REGISTER_FILTER_TYPES, function(RegisterComponentTypesEvent $e) {
+    $e->types[] = ArticleFilter::class;
+});
+```
+
+Filter type classes must implement `craft\webhooks\filters\FilterInterface`:
+
+```php
+use craft\webhooks\filters\FilterInterface;
+use craft\elements\Entry;
+use yii\base\Event;
+
+class ArticleFilter implements FilterInterface
+{
+    public static function displayName(): string
+    {
+        return 'Entry has an “Article” type';
+    }
+
+    public static function show(string $class, string $event): bool
+    {
+        // Only show this filter if the Sender Class is set to 'craft\elements\Entry' 
+        return $class === Entry::class;
+    }
+
+    public static function check(Event $event, bool $value): bool
+    {
+        // Filter basen on whether the entry's type is 'article':
+        /** @var Entry $entry */
+        $entry = $event->sender;
+        return ($entry->type->handle === 'article') === $value;
+    }
+}
+```
+
 #### Sending More Data
 
 If you need more data than what’s in the default POST request payload, you can fill in the “Extra User Attributes”, “Extra Sender Attributes”, and “Extra Event Attributes” fields.