Document how to use the mix commands without install as a lib in one project

[maxmaccari]

Sometimes, I have to access other websites to generate fake CPFs to input what I'm developing and testing.

With this lib, I can just install the package with mix archive.install github ulissesalmeida/cpf and use the command mix cpf.gen to have the CPF generated using only the command line. The same is for the mix cpf.check. And all of this works perfectly.

I think that this way of using it could be documented in the README as well. This tool is very useful for me in this way, and not only as a lib. Perhaps other people may think the same.

[ulissesalmeida]

@maxmaccari Totally agree! Do you wanna create a PR to document this? If you want, feel free to open it.

You can document in this section: https://github.com/ulissesalmeida/cpf#command-line

Or maybe we could move the "command line" section under "Quick Start".