add readme for captcha, and enhanced performance

utils/captcha/README.md

@@ -0,0 +1,45 @@
+# Captcha
+
+an example for use captcha
+
+```
+package controllers
+
+import (
+	"github.com/astaxie/beego"
+	"github.com/astaxie/beego/cache"
+	"github.com/astaxie/beego/utils/captcha"
+)
+
+var cpt *captcha.Captcha
+
+func init() {
+	// use beego cache system store the captcha data
+	store := cache.NewMemoryCache()
+	cpt = captcha.NewWithFilter("/captcha/", store)
+}
+
+type MainController struct {
+	beego.Controller
+}
+
+func (this *MainController) Get() {
+	this.TplNames = "index.tpl"
+}
+
+func (this *MainController) Post() {
+	this.TplNames = "index.tpl"
+
+	this.Data["Success"] = cpt.VerifyReq(this.Ctx.Request)
+}
+```
+
+template usage
+
+```
+{{.Success}}
+<form action="/" method="post">
+	{{create_captcha}}
+	<input name="captcha" type="text">
+</form>
+```