Update development instrucitons in Readme

I **think** that we need to add `github.com` to the path in step #2. There doesn't appear to be a directory located at `$GOPATH/src/exercism/cli`, but there is one located at `$GOPATH/src/github.com/exercism/cli`
Merge pull request #110 from jish/patch-1

Update development instrucitons in Readme