clarify slack token environment and improve documentation

[buritica]

the readme isn't clear about how to get a token for new development, we should improve this

[jzherran]

the readme isn't clear about how to get a token for new development, we should improve this

Hi, I'm going to try to learn about it, and contribute some time to improving it!

I have some questions about that, I'll try to update the readme as a part of the process to set up my local env and also clarify the point you reflect on the issue, but mainly I have a question about the scopes that Leonel uses for its interaction.