Add manual upgrade instructions

[iptizer]

I think it would be a good idea to add manual upgrade instructions in the README.

My story:

• Did not upgrade my Nextcloud in a while. (Yeahh.. I know, but it is how it is.)
• So I upgraded the chart from 2.0.3 to the current version 2.13.3.

This fails, 500er errors are returned. I now probably need to disable maintenance mode or do the manual upgrade steps. But I am totally lacking upgrade instructions.

Why not:

• Add a short comment in the readme only upgrade step by step.
• Provide a few lines of documentation how to execute occ manually within the Helm setup.

[jessebot]

We're definitely happy to include additional docs :) If you'd like to submit a PR, myself or one of our other collaborators can take a look at it.

In the meantime, I'm currently doing investigative work to gather all the upgrade related tickets so we can see commonalities and start coming up with some docs for this, and then link those docs, whenever they're ready, in all the tickets so future people have less of a headache.

I gave a bit of an explanation here on how to upgrade major versions here, but it's not perfect:
#250

Nextcloud's official upgrade process:
https://docs.nextcloud.com/server/latest/admin_manual/maintenance/upgrade.html

Nextcloud docker upgrade process:
https://github.com/nextcloud/docker#update-to-a-newer-version

I am also going through this repo and labeling all the issues related to upgrades with the upgrades label, so we can reference them when drawing up docs.