udpate readme

README.md

@@ -1,9 +1,5 @@
 ## Structured argument parsing for Go
 
-```shell
-go get github.com/alexflint/go-arg
-```
-
 ```go
 var args struct {
 	Foo string
@@ -29,7 +25,7 @@ args.Foo = "default value"
 arg.MustParse(&args)
 ```
 
-### Marking options as required
+### Required options
 
 ```go
 var args struct {
@@ -100,6 +96,12 @@ fmt.Printf("Fetching the following IDs from %s: %q", args.Database, args.IDs)
 Fetching the following IDs from foo: [1 2 3]
 ```
 
+### Installation
+
+```shell
+go get github.com/alexflint/go-arg
+```
+
 ### Rationale
 
 There are many command line argument parsing libraries for Go, including one in the standard library, so why build another?