Fixing bug on cmd

[Menendez6]

While doing the gobl.tin cmd functionality, I was taking this as reference and I came up with 1 bug and I also updated the README:

• If you use the command convert without arguments, the cmd crashes. To fix this I added a condition that when the number of arguments is not 1 or 2 an error displays.
• In the README. documentation for cms used the the file ./test/data/invoice.json which no longer exists. I changed it for ./test/data/invoice-b2b-bare.json to match the response from below. I also changed the command for installing gobl.cfdi because the one before wasn't working: I proposed go install ./cmd/gobl.cfdi, which worked for me.

[cavalle]

Good catch and fix. Thanks!