docs(json): add build tags document.

README.md

@@ -88,7 +88,6 @@ BenchmarkZeus_GithubAll 		| 2000 		| 944234 	| 300688 	| 2648
 - [x] Battle tested
 - [x] API frozen, new releases will not break your code.
 
-
 ## Start using it
 
 1. Download and install it:
@@ -141,6 +140,14 @@ $ curl https://raw.githubusercontent.com/gin-gonic/gin/master/examples/basic/mai
 $ go run main.go
 ```
 
+## Build with [jsoniter](https://github.com/json-iterator/go)
+
+Gin use `encoding/json` as default json package but you can change to [jsoniter](https://github.com/json-iterator/go) by build from other tags.
+
+```
+$ go build -tags=jsoniter .
+```
+
 ## API Examples
 
 ### Using GET, POST, PUT, PATCH, DELETE and OPTIONS