Installation instructions

[GTonehour]

I know this "migra-rebirth" is still in early phases but none of the numerous alternatives seem to have yet reached the stability of migra (even though you deprecated it a few years ago).

This graph shows the relative popularity of those tools: migra still appears to be the most loved, followed by alternatives that don't seem production-ready to me (sqldef fails to handle pg_dumps, pgdiff fails to authenticate, apgdiff can't parse complex dumps, tusker is not an alternative as it relies on migra).

Therefore if the db-diffing part described here is already usable, would you mind adding some install instructions to the README?