Add docs on how to upgrade the image for future active maintainers

[MichelML]

@mcs07 this project is really nice and useful. It's sad that it got unmaintained and is now causing errors when using jointly with newer versions of rdkit (in my case, this is in a django app context).

I would love to contribute and become a maintainer - any way you could help me start or write the docs about the process to upgrade to a newer version of rdkit?

your root image mcs07/rdkit image is easy to update, but this one seems a bit more difficult and is not just a matter of changing the versions in the dockerfile.

Any answer to this issue would help, thanks in advance :+1: