Add to the README that cfssl will be installed in $GOPATH/bin

A simple request since I spent an hour today trying to find out why cfssl didn't install after running `go get -u github.com/cloudflare/cfssl/cmd/cfssl`

The documentation in the README seems to make it seem like cfssl will just work after running `go get -u github.com/cloudflare/cfssl/cmd/cfssl` but i had to ensure I moved the executable into my /bin folder first before I was able to use it globally.

Adding a simple tidbit about this would make life easier for those out there who haven't used Go before.

I can make these changes if you want.