add missing `init` command in README

[ryan-williams]

this seemed necessary for me; i didn't look too hard at whether I'd otherwise missed something, apologies if so

$ goenv env
goenv: unrecognized command env
Goenv provides isolated GOPATH environments for Go projects.

Usage:

    goenv [command] [arguments]

Commands:

    help     get help for a command
    init     initialize a goenv

Use "goenv help [command]" for command-specific information.

$ goenv init env
goenv: initializing...
goenv: wrote activation script at ./env/activate
goenv: done