Clarify modules naming convention

[telatin]

Is your feature request related to a problem? Please describe.

When looking for guidance on modules name I went here and didn't found explicit mention about punctuation. Running nf-core module create bla-bla solved my question, so I thought of explicitly mentioning here too

Describe the solution you'd like
Explicitly mention that punctuation is not allowed, and suggesting that the tool will validate the name.