Added closure documentation for redirect (#700)

docs/security/authentication.md

@@ -756,8 +756,22 @@ When protecting routes for an API, you traditionally return an HTTP response wit
 let protectedRoutes = app.grouped(User.redirectMiddleware(path: "/login?loginRequired=true"))
 ```
 
+The `RedirectMiddleware` object also supports passing a closure that returns the redirect path as a `String` during creation for advanced url handling. For instance, including the path redirected from as query parameter to the redirect target for state management.
+
+```swift
+let redirectMiddleware = User.redirectMiddleware { req -> String in
+  return "/login?authRequired=true&next=\(req.url.path)"
+}
+```
+
 This works similar to the `GuardMiddleware`. Any requests to routes registered to `protectedRoutes` that aren't authenticated will be redirected to the path provided. This allows you to tell your users to log in, rather than just providing a **401 Unauthorized**.
 
+Be sure to include a Session Authenticator before the `RedirectMiddleware` to ensure the authenticated user is loaded before running through the `RedirectMiddleware`.
+
+```swift
+let protectedRoutes = app.grouped([User.SessionAuthenticator(), redirecteMiddleware])
+```
+
 ### Form Log In
 
 To authenticate a user and future requests with a session, you need to log a user in. Vapor provides a `ModelCredentialsAuthenticatable` protocol to conform to. This handles log in via a form. First conform your `User` to this protocol: