Merge pull request #9 from kazuminn/add-godocl-link

add godoc link

README.md

 # subcommands #
 
+[![GoDoc](https://godoc.org/github.com/google/subcommands?status.svg)](https://godoc.org/github.com/google/subcommands)  
 Subcommands is a Go package that implements a simple way for a single command to
 have many subcommands, each of which takes arguments and so forth.