Documentation: add upgrade process to README

[jayaddison]

Todo:

• Follow the documented upgrade instructions here for an existing container, and apply any corrections necessary

[jayaddison]

From searching around in the grocy code: it includes an update.sh script: not all of that's relevant (or straightforward) in a container environment, but perhaps it'll be possibly to unify an approach at some point so that the same steps are followed regardless of container/non-container deployment.

[jayaddison]

I practised using these guidelines to upgrade a local containerized Grocy instance from v3.3.1-6 to v3.3.2-0, and that seems to have succeeded ✅

While setting up the test containers for v3.3.1-6, I did skip some of the 'official' Docker volume backup/restore steps (because I think it's ridiculous to run an entire container simply to copy some files, as mentioned in #196) -- and after doing that, ran into some login-time errors due to stale viewcache data. Removing the viewcache directory, and ensuring the correct file permissions solved the problem.