release policy in the readme

A few open questions:

[x] how do we name versions in-between 2023.0 and 2023.1 ? I guess just by incrementing point version 2023.0.1, 2023.0.2, ..., 2023.0.329 ?
[ ] how to we identify migrations, since we don't know the name of the version at the time of the PR (as it depends of the merge order). Maybe we could just use a number (delta_2_change-description.sql, delta_3_other-change-description.sql, ...), starting at 2 since the current on is 1.6.0, and avoiding any reference to 2023 since there is no correlation between deltas and releases ? Also we need to ensure it's unambiguously ordered (worth a check in CI that a PR doesn't introduce older migrations IMO)

teksi / wastewater_old