udpate readme

README.md

@@ -1,4 +1,4 @@
-# Argument parsing for Go
+## Structured argument parsing for Go
 
 ```go
 var args struct {
@@ -11,7 +11,7 @@ fmt.Println(args.Foo, args.Bar)
 
 ```shell
 $ ./example --foo=hello --bar
-hello True
+hello true
 ```
 
 ### Default values
@@ -41,7 +41,6 @@ arg.MustParse(&args)
 var args struct {
 	Input   string   `arg:"positional"`
 	Output  []string `arg:"positional"`
-	Verbose bool
 }
 arg.MustParse(&args)
 fmt.Println("Input:", input)
@@ -96,3 +95,13 @@ fmt.Printf("Fetching the following IDs from %s: %q", args.Database, args.IDs)
 ./example -database foo -ids 1 2 3
 Fetching the following IDs from foo: [1 2 3]
 ```
+
+### Rationale
+
+There are many command line argument parsing libraries for golang, including one in the standard library, so why build another?
+
+The shortcomings of the `flag` library that ships in the standard library are well known. Positional arguments must preceed options, so `./prog x --foo=1` does what you expect but `./prog --foo=1 x` does not. Bool arguments must have explicit values, so `./prog -debug=1` sets debug to true but `./myprog -debug` does not.
+
+Many third-party argument parsing libraries are geared for writing sophisticated command line interfaces. The excellent `codegangsta/cli` is perfect for implementing sophisticated command line tools, with multiple sub-commands and nested flags, but is probably overkill for a simple script with a handful of flags.
+
+The main idea behind `go-arg` is that golang already has an excellent way to describe named data structures using structs, so there is no need to develop more levels of abstraction on top of this. Instead of one API to specify which arguments your program accepts, and then another API to get the values of those arguments, why not just describe both with a single struct?