Add and fix the explanation of `HandleContext` (#1371)

Reference this issue #1323 

1. There isn't any eg about `HandleContext`  
2. The `c.Request.Path`  of `HandleContext` Comment  is not right   

Based on the above two points, I pull this request.  
If you think it's unnecessary, I will close this.  
Thx.

README.md

@@ -1082,14 +1082,26 @@ Gin allow by default use only one html.Template. Check [a multitemplate render](
 
 ### Redirects
 
-Issuing a HTTP redirect is easy:
+Issuing a HTTP redirect is easy. Both internal and external locations are supported.
 
 ```go
 r.GET("/test", func(c *gin.Context) {
 	c.Redirect(http.StatusMovedPermanently, "http://www.google.com/")
 })
 ```
-Both internal and external locations are supported.
+
+
+Issuing a Router redirect, use `HandleContext` like below.
+
+``` go
+r.GET("/test", func(c *gin.Context) {
+    c.Request.URL.Path = "/test2"
+    r.HandleContext(c)
+})
+r.GET("/test2", func(c *gin.Context) {
+    c.JSON(200, gin.H{"hello": "world"})
+})
+```
 
 
 ### Custom Middleware

gin.go

@@ -329,7 +329,7 @@ func (engine *Engine) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 }
 
 // HandleContext re-enter a context that has been rewritten.
-// This can be done by setting c.Request.Path to your new target.
+// This can be done by setting c.Request.URL.Path to your new target.
 // Disclaimer: You can loop yourself to death with this, use wisely.
 func (engine *Engine) HandleContext(c *Context) {
 	c.reset()