Add documentation for Preconditions, Aliases, Command Structure

Resolves #223

docs/guides/samples/require_context.cs

@@ -0,0 +1,11 @@
+[Module]
+public class InfoModule
+{
+    // Constrain this command to Guilds
+    [RequireContext(ContextType.Guild)]
+    public async Task Whois(IMessage msg, IGuildUser user) { }
+
+    // Constrain this command to either Guilds or DMs
+    [RequireContext(ContextType.Guild | ContextType.DM)]
+    public async Task Info(IMessage msg) { }
+}