docs: Add Webhook example. (#1241)

* Add Webhook example.

* Fix spacing.

* Update to use webhook URL ctor.

samples/04_webhook_client/04_webhook_client.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp2.2</TargetFramework>
+    <RootNamespace>_04_webhook_client</RootNamespace>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Discord.Net.Webhook\Discord.Net.Webhook.csproj" />
+  </ItemGroup>
+
+</Project>

samples/04_webhook_client/Program.cs

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+using Discord;
+using Discord.Webhook;
+using System.Threading.Tasks;
+
+namespace _04_webhook_client
+{
+    // This is a minimal example of using Discord.Net's Webhook Client
+    // Webhooks are send-only components of Discord that allow you to make a POST request
+    // To a channel specific URL to send a message to that channel.
+    class Program
+    {
+        static void Main(string[] args)
+            => new Program().MainAsync().GetAwaiter().GetResult();
+
+        public async Task MainAsync()
+        {
+            // The webhook url follows the format https://discordapp.com/api/webhooks/{id}/{token}
+            // Because anyone with the webhook URL can use your webhook
+            // you should NOT hard code the URL or ID + token into your application. 
+            using (var client = new DiscordWebhookClient("https://discordapp.com/api/webhooks/123/abc123"))
+            {
+                var embed = new EmbedBuilder
+                {
+                    Title = "Test Embed",
+                    Description = "Test Description"
+                };
+
+                // Webhooks are able to send multiple embeds per message
+                // As such, your embeds must be passed as a collection. 
+                await client.SendMessageAsync(text: "Send a message to this webhook!", embeds: new[] { embed.Build() });
+            }
+        }
+    }
+}