Documentation and Build Updates

[vicchi]

• Minor additions to the README to call out the dependency on togeojson (commented out in `update.sh)
• Tighten up update.sh to:
  • Pass all GeoJSON files through geojson-rewind to ensure RFC-7946 winding order (you'd think Mapbox would do this in togeojson but no ...)
  • Skip trying to process non-geographic postcode areas :-
  • BF - British Forces Post Office (BFPO)
  • BX - reserved solely for non-geographic addresses
  • GIR - reserved for Girobank
  • Skip trying to process non-UK postcode areas (which have no associated KML file) :-
  • GY, JE - Guernsey and Jersey, managed by Guernsey Post and Jersey Post respectively
  • IM - Isle of Man - managed by the Isle of Man Post Office
  • Skip trying to process empty KML files, which basically means BT in Northern Ireland
• Minor update to the data for CR as of September 2020

[missinglink]

better late than never, I've preserved your commits, updated the data for 2021 and merged them into master, thanks!