Update README with all the useful commands (scripts) available

[kowh-ai]

There are a number of very useful commands available to run from the host machine. These should be included in the README file for this repo

The commands will be along the lines of:

docker compose -f docker-compose.dev.yml exec <script options> ckan-dev ckan <ckan options> <ckan command>

[wardi]

even better if those could be written as

./devexec ckan <ckan options> <ckan command>

[duttonw]

per #193 can we also cross link to https://github.com/ckan/ckan-docker-base as they are siblings/twins and work together.