Add docs for the event listeners

[Jose M. Valera Reales]

Add documentation for the internal gacela event-listener system.

[Fabrizio Fasanando]

Hello 👋

I see that changes related to this issue where merged to master in the #36 PR. This issue should be close?

[Jose M. Valera Reales]

Yes 🙌 Thanks for reaching out. We can close this issue.