docs: add docs on missing redirect.callback option (#128)

docs/options.md

@@ -9,14 +9,16 @@ Default:
 ```js
 redirect: {
   login: '/login',
-  logout: '/'
+  logout: '/',
+  callback: '/login',
   home: '/'
 }
 ```
 
 * `login`: User will be redirected to this path if *login is required*.
-* `logout`: User will be redirected to this path if *after logout current route is protected*.
-* `user`: User will be redirect to this path *after login*. (`rewriteRedirects` will rewrite this path)
+* `logout`: User will be redirected to this path if *after logout, current route is protected*.
+* `callback`: User will be redirect to this path by the identity provider *after login*. (Should match configured `Allowed Callback URLs` (or similar setting) in your app/client with the identity provider) 
+* `user`: User will be redirected to this path *after login and callback is processed*. (`rewriteRedirects` will rewrite this path)
 
 Each redirect path can be disabled by setting to `false`.
 Also you can disable all redirects by setting `redirect` to `false`