Add documentation

[Jeremy Tuloup]

For now we point to the PlasmaBio documentation since it contains screenshots and explanations: https://docs.plasmabio.org/en/latest/environments/index.html

Later we might want to move part of this documentation to the tljh-repo2docker repo if it doesn't have anything specific to PlasmaBio.

[Pierre Poulain]

We could separate the doc in two parts.

• one part, in the tljh-repo2docker repo and dedicated to the later.
• another part, in the plasmabio repo, more related to PlasmaBio with many links and references to the doc of tljh-repo2docker

[Jeremy Tuloup]

That sounds good.

The User Environments section in the docs is quite self contained and could be moved here.