Add hints on how to install this package

[Giuseppe Crinò]

README is showing no hints on how to install the package. After go get github.com/cloudflare/gokey there's no binary in $GOPATH/bin.

[Ignat Korchagin]

Right, that is because go get github.com/cloudflare/gokey will get you the code, not the cmdline tool.

To get the tool you need go get github.com/cloudflare/gokey/cmd/gokey

I'll update the README.

Thanks