add README

README.md

@@ -0,0 +1,89 @@
+Argument parsing for Go.
+
+```golang
+var args struct {
+	Foo  string
+	Bar  bool
+}
+arg.MustParse(&args)
+fmt.Println(args.Foo)
+fmt.Println(args.Bar)
+```
+
+```bash
+$ ./example --foo=hello --bar
+hello
+True
+```
+
+Setting defaults values:
+
+```golang
+var args struct {
+	Foo string
+	Bar bool
+}
+args.Foo = "default value"
+arg.MustParse(&args)
+```
+
+Marking options as required
+
+```golang
+var args struct {
+	Foo string `arg:"required"`
+	Bar bool
+}
+arg.MustParse(&args)
+```
+
+Positional argument:
+
+```golang
+var args struct {
+	Input   string   `arg:"positional"`
+	Output  []string `arg:"positional"`
+	Verbose bool
+}
+arg.MustParse(&args)
+fmt.Println("Input:", input)
+fmt.Println("Output:", output)
+```
+
+```
+$ ./example src.txt x.out y.out z.out
+Input: src.txt
+Output: [x.out y.out z.out]
+```
+
+Usage strings:
+```bash
+$ ./example -h
+usage: [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--help] INPUT [OUTPUT [OUTPUT ...]] 
+
+positional arguments:
+  input
+  output
+
+options:
+--verbose, -v            verbosity level
+--dataset DATASET        dataset to use
+--optimize OPTIMIZE, -O OPTIMIZE
+                         optimization level
+--help, -h               print this help message
+```
+
+Options with multiple values:
+```
+var args struct {
+	Database string
+	IDs      []int64
+}
+arg.MustParse(&args)
+fmt.Printf("Fetching the following IDs from %s: %q", args.Database, args.IDs)
+```
+
+```bash
+./example -database foo -ids 1 2 3
+Fetching the following IDs from foo: [1 2 3]
+```