Added How To Reject Client Connections guide

docs/docs/how-to/reject-client-connections.md

@@ -0,0 +1,30 @@
+# Rejecting Client Connections
+
+SpacetimeDB provides a way to disconnect a client during a client connection attempt.
+:::server-rust
+In Rust, if we returned and error (or a panic) during the `client_connected` reducer, the client would be disconnected.
+
+Here is a simple example where the server module throws an error for all incoming client connections.
+```rust
+#[reducer(client_connected)]
+// Called when a client connects to a SpacetimeDB database server
+pub fn client_connected(ctx: &ReducerContext) {
+    return Err("All incoming client connections are being rejected. Normally you'd want to only throw an error after your validation logic indicates the client is not authorized.".into());
+}
+```
+:::
+:::server-csharp
+In C#, if we throw an exception during the `ClientConnected` reducer, the client would be disconnected.
+
+Here is a simple example where the server module throws an error for all incoming client connections.
+```csharp
+[Reducer(ReducerKind.ClientConnected)]
+// Called when a client connects to a SpacetimeDB database server
+public static void ClientConnected(ReducerContext ctx)
+{
+    throw new Exception("All incoming client connections are being rejected. Normally you'd want to only throw an error after your validation logic indicates the client is not authorized.");
+}
+```
+:::
+
+From the client's perspective, this disconnection behavior is currently undefined, but they will be disconnected from the server's perspective.