Describe the pull request

Adds fail2ban setup instrctions document.

I spent some time figuring out how to setup fail2ban to protect this gogs docker, turned out very tricky.

I hope the notes save time for others.

Fail2ban in public server is quite necessary, many ssh worm will try to access every port on the internet, when they found one, a big concurrent traffic attack could come after.

Checklist

• [V] I agree to follow the Code of Conduct by submitting this pull request.
• [V] I have read and acknowledge the Contributing guide.
• [V] I have added test cases to cover the new code or have provided the test plan.