Add a description to be opt-out in the issue created

[francoto]

Maintainer contacted with the issue generated may not be willing to receive anymore review from the bot.

The generated issue should explain how they can unsubscribe from the bot by simply adding a "unsubsribe" reply in the issue generated

[francoto]

add explicit message to opt out in the created issue using the "unsubscribed" keyword

[francoto]

It was actually already in the body of the evalaution but just before the list of warnings and pitfalls so it wasn't easy to notice it. I put it in the bottom of the issue to be more visible.