Add common use cases to documentation

[fabriciojs]

There are two recurring use cases that we often rise up support from Kool users.

• [ ] How to give a running container the same SSH access your host user have (sharing SSH keys) - consider Snapcraft issue
• [ ] How to customize a Kool image? (using a Dockerfile, extending it)
• [ ] How to get up-to-date images?
• [ ] Best practices? Like, never using sudo kool.
• [ ] Current known steps and best practices to run multiple kool environments simultaneously #310
• [ ] Document COMPOSE_FILE and possibly other Docker Compose environment variables that give flexibility (and sample use case). #294 (also covers https://github.com/kool-dev/kool/issues/140 use case)