feat(popular-topics): Add webhooks section in threads page (discordjs…

…#932)

guide/popular-topics/threads.md

@@ -165,4 +165,18 @@ const thread = channel.threads.cache.find(x => x.name === 'food-talk');
 await thread.members.remove('140214425276776449');
 ```
 
+## Sending messages to threads with webhooks
+
+It is possible for a webhook built on the parent channel to send messages to the channel's threads. For the purpose of this example, it is assumed a single webhook already exists for that channel. If you wish to learn more about webhooks, see our [webhook guide](/popular-topics/webhooks.md).
+
+```js
+const webhooks = await channel.fetchWebhooks();
+const webhook = webhooks.first();
+
+await webhook.send({
+	content: 'Look ma! I\'m in a thread!',
+	threadId: '123456789012345678',
+});
+```
+
 And that's it! Now you know all there is to know on working with threads using discord.js!