docs: added comment in doc.go (#4274)

doc.go

@@ -2,5 +2,21 @@
 Package gin implements a HTTP web framework called gin.
 
 See https://gin-gonic.com/ for more information about gin.
+
+Example:
+
+	package main
+
+	import "github.com/gin-gonic/gin"
+
+	func main() {
+		r := gin.Default()
+		r.GET("/ping", func(c *gin.Context) {
+			c.JSON(200, gin.H{
+				"message": "pong",
+			})
+		})
+		r.Run() // listen and serve on 0.0.0.0:8080
+	}
 */
 package gin // import "github.com/gin-gonic/gin"