udpate readme

README.md

@@ -1,5 +1,9 @@
 ## Structured argument parsing for Go
 
+```shell
+go get github.com/alexflint/go-arg
+```
+
 ```go
 var args struct {
 	Foo string
@@ -14,12 +18,6 @@ $ ./example --foo=hello --bar
 hello true
 ```
 
-### Installation
-
-```shell
-go get github.com/alexflint/go-arg
-```
-
 ### Default values
 
 ```go