add session's readme

+sessionmanager
+==============
+
+sessionmanager is a golang session manager. It can use session for many providers.Just like the `database/sql` and `database/sql/driver`.
+
+##How to install
+
+	go get github.com/astaxie/beego/session
+
+
+##how many providers support
+Now this sessionmanager support memory/file/redis/mysql
+
+
+
+##How do we use it?
+
+first you must import it
+
+
+	import (
+		"github.com/astaxie/beego/session"
+	)
+
+then in you web app init the globalsession manager
+	
+	var globalSessions *session.Manager
+
+use memory as providers:
+
+	func init() {
+		globalSessions, _ = session.NewManager("memory", "gosessionid", 3600,"")
+		go globalSessions.GC()
+	}
+
+use mysql as providers,the last param is the DNS, learn more from [mysql](https://github.com/Go-SQL-Driver/MySQL#dsn-data-source-name): 
+
+	func init() {
+		globalSessions, _ = session.NewManager("mysql", "gosessionid", 3600,"username:password@protocol(address)/dbname?param=value")
+		go globalSessions.GC()
+	}
+
+use file as providers,the last param is the path where to store the file:
+
+	func init() {
+		globalSessions, _ = session.NewManager("file", "gosessionid", 3600,"./tmp")
+		go globalSessions.GC()
+	}
+
+use redis as providers,the last param is the redis's conn address:
+
+	func init() {
+		globalSessions, _ = session.NewManager("redis", "gosessionid", 3600,"127.0.0.1:6379")
+		go globalSessions.GC()
+	}
+
+at last in the handlerfunc you can use it like this
+
+	func login(w http.ResponseWriter, r *http.Request) {
+		sess := globalSessions.SessionStart(w, r)
+		defer sess.SessionRelease()
+		username := sess.Get("username")
+		fmt.Println(username)
+		if r.Method == "GET" {
+			t, _ := template.ParseFiles("login.gtpl")
+			t.Execute(w, nil)
+		} else {
+			fmt.Println("username:", r.Form["username"])
+			sess.Set("username", r.Form["username"])
+			fmt.Println("password:", r.Form["password"])
+		}
+	}
+	
+
+
+##How to write own provider
+When we develop a web app, maybe you want to write a provider because you must meet the requirements.
+
+Write a provider is so easy. You only define two struct type(Session and Provider),which satisfy the interface definition.Maybe The memory provider is a good example for you.
+
+	type SessionStore interface {
+		Set(key, value interface{}) error //set session value
+		Get(key interface{}) interface{}  //get session value
+		Delete(key interface{}) error     //delete session value
+		SessionID() string                //back current sessionID
+		SessionRelease()                  // release the resource
+	}
+	
+	type Provider interface {
+		SessionInit(maxlifetime int64, savePath string) error
+		SessionRead(sid string) (SessionStore, error)
+		SessionDestroy(sid string) error
+		SessionGC()
+	}
+
+##LICENSE
+
+BSD License http://creativecommons.org/licenses/BSD/
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