Github Actions

[edzer]

now running, still todo:

• [ ] render python branch somewhere else (netlify?)
• [ ] run on PRs
• [x] Python branch in cron job
• [x] add to README that this is a daily built, unchecked
• [x] install spatialreg from CRAN, once released
• [x] put errata somewhere, with link

[edzer]

The RPostgres() examples in Chapter 9 don't run here, neither the docker nor GA don't have PostgreSQL/PostGIS set up.