suppress deprecation message during swag install (#59)

Author: salprima

README.md

@@ -14,7 +14,7 @@ echo middleware to automatically generate RESTful API documentation with Swagger
 1. Add comments to your API source code, [See Declarative Comments Format](https://github.com/swaggo/swag#declarative-comments-format).
 2. Download [Swag](https://github.com/swaggo/swag) for Go by using:
 ```sh
-$ go get github.com/swaggo/swag/cmd/swag
+$ go get -d github.com/swaggo/swag/cmd/swag
 ```
 3. Run the [Swag](https://github.com/swaggo/swag) in your Go project root folder which contains `main.go` file, [Swag](https://github.com/swaggo/swag) will parse comments and generate required files(`docs` folder and `docs/doc.go`).
 ```sh_ "github.com/swaggo/echo-swagger/v2/example/docs"