Add docs about version update

[svx]

We should add docs about how to update to a newer version

something like:

• create dir with version nr as name
• add tag

and so on

[svx]

@avoinea I will create a place for this under /docs and I will exclude it from sphinx build and from papyrus.
That means we will have it here in /docs on GitHub, but it will not show up on docs.plone.org.
Reason for that is I would like to keep the docs on docs.plone.org only about docs about docker, not on how we work on the container itself.
Is that OK with you ?

[avoinea]

@svx I'm OK with this

[svx]

Great !

[svx]

@avoinea I guess we missed to write this part of the docs :)

[avoinea]

:)