update validation readme

recipes/validation.md

@@ -2,6 +2,8 @@
 
 tRPC works out-of-the-box with yup/superstruct/zod/myzod/custom validators. Learn more about input validation [here](https://trpc.io/docs/router#input-validation).
 
+## Input Validation
+
 ```ts
 // ~/server/trpc/index.ts
 import { z } from 'zod'
@@ -21,3 +23,25 @@ export const router = trpc
     },
   })
 ```
+
+## Output Validation
+
+```ts
+// ~/server/trpc/index.ts
+import { z } from 'zod'
+
+export const router = trpc
+  .router()
+  .query('hello', {
+    // validate output with Zod
+    output: z.object({
+      greeting: z.string()
+    }),
+    // expects return type of { greeting: string }
+    resolve() {
+      return {
+        greeting: 'hello!',
+      }
+    },
+  })
+```