Update README.md to include new workflows and info

[CodeGat]

The README.md has come out of sync with the overall structure of the repository. Modify it so it makes sense again.

Changes required:

• Write about the generic ci.yml, ci-closed.yml, ci-comment.yml, cd.yml and how they hook into the deploy-*/undeploy-* workflows
• Note the sanitzation of model repo names for spack, as noted in Model names replace . with p for ACCESS-ESM1.5 #68 (comment)
• Update the inputs/outputs of each reusable workflow
• Note what is inside the new config and tools directories
• Write up a generic How To guide for adding CI/CD, secrets, variables etc. for the creation of a new model

References #68