Document geodiff PostGIS connection syntax

[volcan01010]

Summary

As a PostGIS user, I would like documentation on connecting to a PostGIS schema and running geodiff, so that I can use it to obtain and apply diffs there.

Description

The documentation explains how to compare GeoPackages and get a list of changes using Python: https://github.com/MerginMaps/geodiff#using-python-module

I would like to perform the same actions against a PostGIS database, but the connection syntax isn't explained.
There are PostGIS connections used in some of the Python unit tests but these use a test class.

Acceptance criteria

• README and/or other documentation updated to show how to connect to a PostGIS database.

[volcan01010]

The syntax used in the test system is here:

geodiff/pygeodiff/tests/testutils.py

Line 148 in e71dfe1

class GeoDiffTestSqlDb:

This is only in a test class. Is there no "official" way to connect to PostGIS.

Our use case is one main schema and users working in teams on separate schemas for weeks at a time, then merging back to main.

This line in db-link suggests that the geodiff command line can be used by passing driverandconninfo` strings.

https://github.com/MerginMaps/db-sync/blob/d99cb80a8b05549b84577656e5f8a8b1766cccaf/dbsync.py#L213