docs: change example in readme to slash command (#6250)

Co-authored-by: Noel <buechler.noel@outlook.com>

README.md

@@ -41,6 +41,35 @@ npm install discord.js
 
 ## Example usage
 
+First, we need to register a slash command against the Discord API:
+```js
+const { REST } = require('@discordjs/rest');
+const { Routes } = require('discord-api-types/v9');
+
+const commands = [{
+  name: 'ping',
+  description: 'Replies with Pong!'
+}]; 
+
+const rest = new REST({ version: '9' }).setToken('token');
+
+(async () => {
+    try {
+      console.log('Started refreshing application (/) commands');
+    
+      await rest.put(
+        Routes.applicationGuildCommands(CLIENT_ID, GUILD_ID),
+        { body: commands },
+      );
+    
+      console.log('Sucessfully reloaded application (/) commands.');
+    } catch (error) {
+      console.error(error);
+    }
+})()
+```
+
+Afterwards we can create a quite simple example bot:
 ```js
 const { Client, Intents } = require('discord.js');
 const client = new Client({ intents: [Intents.FLAGS.GUILDS, Intents.FLAGS.GUILD_MESSAGES] });
@@ -49,9 +78,11 @@ client.on('ready', () => {
   console.log(`Logged in as ${client.user.tag}!`);
 });
 
-client.on('messageCreate', message => {
-  if (message.content === 'ping') {
-    message.channel.send('pong');
+client.on('interactionCreate', async interaction => {
+  if (!interaction.isCommand()) return;
+
+  if (interaction.commandName === 'ping') {
+    await interaction.reply('Pong!');
   }
 });