Provide custom options of TrustedPlatform for another CDN services (#2906)

* refine TrustedPlatform and docs

* refactor for switch

* refactor switch to if statement

README.md

@@ -78,6 +78,7 @@ Gin is a web framework written in Go (Golang). It features a martini-like API wi
     - [http2 server push](#http2-server-push)
     - [Define format for the log of routes](#define-format-for-the-log-of-routes)
     - [Set and get a cookie](#set-and-get-a-cookie)
+  - [Don't trust all proxies](#don't-trust-all-proxies)
   - [Testing](#testing)
   - [Users](#users)
 
@@ -2236,6 +2237,34 @@ func main() {
 }
 ```
 
+**Notice:** If you are using a CDN service, you can set the `Engine.TrustedPlatform`
+to skip TrustedProxies check, it has a higher priority than TrustedProxies. 
+Look at the example below:
+```go
+import (
+	"fmt"
+
+	"github.com/gin-gonic/gin"
+)
+
+func main() {
+
+	router := gin.Default()
+	// Use predefined header gin.PlatformXXX
+	router.TrustedPlatform = gin.PlatformGoogleAppEngine
+	// Or set your own trusted request header for another trusted proxy service
+	// Don't set it to any suspect request header, it's unsafe
+	router.TrustedPlatform = "X-CDN-IP"
+
+	router.GET("/", func(c *gin.Context) {
+		// If you set TrustedPlatform, ClientIP() will resolve the
+		// corresponding header and return IP directly
+		fmt.Printf("ClientIP: %s\n", c.ClientIP())
+	})
+	router.Run()
+}
+```
+
 ## Testing
 
 The `net/http/httptest` package is preferable way for HTTP testing.