Add docs for protecting the documentation routes

When @fastify/swagger-ui was part of @fastify/swagger there used to be documentation for protecting the documentation routes, but that wasn't transferred over when [it was removed](fastify/fastify-swagger@f15bebd#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5L689) it seems.

I know I've spent a bit of time searching for how to do this, stumbling upon fastify/fastify-swagger#466 and getting confused when the docs are not there anymore in the current HEAD.

I think it would be beneficial to add this to the fastify-swagger-ui docs since it's a very common use case.

README.md

@@ -225,6 +225,35 @@ await fastify.register(require('@fastify/swagger-ui'), {
 })
 ```
 
+#### Protect your documentation routes
+
+You can protect your documentation by configuring an authentication hook.
+Here is an example using the [`@fastify/basic-auth`](https://github.com/fastify/fastify-basic-auth) plugin:
+
+##### Example
+```js
+const fastify = require('fastify')()
+
+fastify.register(require('@fastify/swagger'))
+
+await fastify.register(require('@fastify/basic-auth'), {
+  validate (username, password, req, reply, done) {
+    if (username === 'admin' && password === 'admin') {
+      done()
+    } else {
+      done(new Error('You can not access'))
+    }
+  },
+  authenticate: true
+})
+
+await fastify.register(require('@fastify/swagger-ui', {
+  uiHooks: {
+    onRequest: fastify.basicAuth
+  }
+})
+```
+
 <a name="license"></a>
 ## License