Added minimal example of using kerberos

yesod-auth/kerberos.hs

@@ -0,0 +1,42 @@
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE QuasiQuotes #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+import Yesod
+import Yesod.Auth
+import Yesod.Form
+import Yesod.Auth.Kerberos
+
+data Kerberos = Kerberos
+type Handler = GHandler Kerberos Kerberos
+
+mkYesod "Kerberos" [parseRoutes|
+/ RootR GET
+/after AfterLoginR GET
+/auth AuthR Auth getAuth
+|]
+
+getRootR :: Handler ()
+getRootR = redirect RedirectTemporary $ AuthR LoginR
+
+getAfterLoginR :: Handler RepHtml
+getAfterLoginR = defaultLayout $ return ()
+
+instance Yesod Kerberos where
+    approot _ = "http://localhost:3000"
+
+instance YesodAuth Kerberos where
+    type AuthId Kerberos = String
+    loginDest _ = AfterLoginR
+    logoutDest _ = AuthR LoginR
+    getAuthId _ = do
+        liftIO $ putStrLn "getAuthId"
+        return $ Just "foo"
+    authPlugins = [authKerberos]
+
+instance RenderMessage Kerberos FormMessage where
+    renderMessage _ _ = defaultFormMessage
+
+main :: IO ()
+main = warpDebug 3000 Kerberos