Document the upgrade process

[maelvls]

Right now, users have no way to upgrade from an earlier version they installed using either click-to-deploy or using the CLI instructions.

This piece of work is about figuring out how to guide users through the upgrade process, add these instructions to the README.md for both CLI and Click-to-deploy users. I looked at unleash-hosted-gcp-marketplace but they don't mention the upgrade process.

Here is the distinction I do between "upgrade" and "update" in the context of this issue:

Term	Description
update	change the tag used by the GCM app, e.g. 1.1.0-gcm.8 → 1.3.1-gcm.1
upgrade	steps that come before and after an update of the GCM app, including upgrading.md

[maelvls]

For now, I document the upgrade process in the GitHub Release page, e.g. the 1.4.0-gcm.0 release has an "Upgrade instructions" section.

I will close this issue for now.