Document our policies around upgrade paths

[lfrancke]

We decided that - for now - we will only support upgrading from one version to the next. Skipping an intermediary release is not currently supported.

This policy needs to be documented. It doesn't have to be much.

Can be something like this

= Upgrade Policy

Skipping SDP releases while upgrading is not currently supported.
The only upgrade path we support is from one version to the next.