Update README.md (#1019)

* Update README.md

Major update:

- Refer to `just-the-docs-template` for getting started.
- Avoid duplication of details provided in the template README.
- Distinguish between theme users and theme developers.
- Augment the items for submitting code changes
- Make the marked-up text easier to read and edit by using short-cut link references.

(Not yet checked on GitHub.)

* Restore docs for using JTD as a gem