Simple instructions for Joplin server upgrade

[dpoulton]

@KajWiik

You have to use absolute version, latest will not bring the latest one .

I believe that the latest tag is being retained for actual release versions. Everything at the moment, as far as the server software is concerned, is a beta.

@laurent

'"latest" will indeed be the stable release, but there was an issue when I've setup building the Docker image from CI, and a 2.0.x version ended up as latest. I've decided to leave it as it is because even 1.x was a beta version anyway. At this point, all versions are beta.

We're quite close to the first stable though, and from then on the latest tag will be used properly to denote a stable release