Information about documentation in docs

- Read the docs builds here: https://empiarreader.readthedocs.io/en/latest/
- PRs make a preview of how the docs will look on RtD
- Build docs by hand in usual way for sphinx (NB these instructions may be wrong):
- make sure `pip install empiarreader[doc]` is installed (or `pip install .[doc]` if installing locally)
- cd into `docs`
- `make html`
- open index.html in the build directory
- TODO - note the above in some developer docs