docs(react-start): serialization adapters

Author: m9tdev

docs/start/framework/react/guide/server-functions.md

@@ -270,6 +270,31 @@ export default defineConfig({
 })
 ```
 
+### Custom serialization adapters
+
+You can create custom serialization adapters to handle complex types that can't be serialized by default.
+
+Example:
+
+```ts
+// src/start.ts
+import { createStart } from "@tanstack/react-start";
+import { createSerializationAdapter } from "@tanstack/react-router";
+
+const bigIntAdapter = createSerializationAdapter({
+  key: "bigint",
+  test: (value: unknown): value is bigint => typeof value === "bigint",
+  toSerializable: (bigInt) => bigInt.toString(),
+  fromSerializable: (value) => BigInt(value),
+});
+
+export const startInstance = createStart(() => {
+  return {
+    serializationAdapters: [bigIntAdapter],
+  };
+});
+```
+
 ---
 
 > **Note**: Server functions use a compilation process that extracts server code from client bundles while maintaining seamless calling patterns. On the client, calls become `fetch` requests to the server.