Description of the basic procedure to keep the local database in sync with OSM

Added here a basic description of the standard procedure to keep the local PostgreSQL/PostGIS database in sync with OpenStreetMap as data is progressively updated to OSM.

All openstreetmap.org urls have been changed to https.

Documentation updated to openstreetmap-carto tag v4.8.0, commit 5479f91 performed on March 7, 2018.