Add postgres example code

Cookbook.md

@@ -37,6 +37,7 @@ For non-Yesod code, see also [Snippets](https://github.com/yesodweb/yesod-cookbo
 * [Lens for existing site](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Lens-for-existing-site.md)
 * [Connecting to an additional existing database](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Connecting-to-an-additional-existing-database.md)
 * [Example MySQL Connection code](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Example-MySQL-Connection-code.md)
+* [Example Postgres Connection code](./cookbook/postgres-example-code.md)
 * [Activate foreign key checking in Sqlite](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Activate-foreign-key-checking-in-Sqlite.md)
 * [Adding Seed data to Scaffolded Site](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Adding-Seed-Data-to-Scaffolded-Site.md)
 * [Handling exception in persistence](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Handling-Persistence-Exception.md)

cookbook/postgres-example-code.md

@@ -0,0 +1,56 @@
+Example Posgres Persistent code:
+
+``` haskell
+#!/usr/bin/env stack
+{- stack
+     --resolver lts-7.3
+     --install-ghc
+     runghc
+     --package persistent
+     --package persistent-postgresql
+     --package persistent-template
+     --package network
+     --package mtl
+-}
+
+
+{-# LANGUAGE EmptyDataDecls #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE QuasiQuotes #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeFamilies #-}
+
+import Control.Monad.IO.Class (liftIO)
+import Control.Monad.Logger (runStderrLoggingT)
+import Database.Persist
+import Database.Persist.Postgresql
+import Database.Persist.TH
+
+share
+  [mkPersist sqlSettings, mkMigrate "migrateAll"]
+  [persistLowerCase|
+Peerson
+    name String
+    age Int
+    UniqueAge age
+    deriving Show
+|]
+
+connStr = "host=localhost dbname=test user=postgres password=postgres port=5432"
+
+main :: IO ()
+main =
+  runStderrLoggingT $
+  withPostgresqlPool connStr 10 $
+  \pool ->
+     liftIO $
+     do flip runSqlPersistMPool pool $
+          do runMigration migrateAll
+             johnId <- insert $ Peerson "John Doe" 35
+             liftIO $ print johnId
+             return ()
+```