feat(engine): add trustedproxies and remoteIP (#2632)

Co-authored-by: Søren L. Hansen <soren@linux2go.dk>
Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>
Co-authored-by: thinkerou <thinkerou@gmail.com>
Co-authored-by: Javier Provecho Fernandez <javiertitan@gmail.com>

README.md

@@ -2124,6 +2124,39 @@ func main() {
 }
 ```
 
+## Don't trust all proxies
+
+Gin lets you specify which headers to hold the real client IP (if any),
+as well as specifying which proxies (or direct clients) you trust to
+specify one of these headers.
+
+The `TrustedProxies` slice on your `gin.Engine` specifes network addresses or
+network CIDRs from where clients which their request headers related to client
+IP can be trusted. They can be IPv4 addresses, IPv4 CIDRs, IPv6 addresses or
+IPv6 CIDRs.
+
+```go
+import (
+	"fmt"
+
+	"github.com/gin-gonic/gin"
+)
+
+func main() {
+
+	router := gin.Default()
+	router.TrustedProxies = []string{"192.168.1.2"}
+
+	router.GET("/", func(c *gin.Context) {
+		// If the client is 192.168.1.2, use the X-Forwarded-For
+		// header to deduce the original client IP from the trust-
+		// worthy parts of that header.
+		// Otherwise, simply return the direct client IP
+		fmt.Printf("ClientIP: %s\n", c.ClientIP())
+	})
+	router.Run()
+}
+```
 
 ## Testing