Add Contribution Instructions

[stevejpurves]

Update the readme with guidelines for contributing templates and template format specifics

[tavin]

I am especially mystified about how to write a site template (or just modify https://github.com/myst-templates/book-theme).

[rowanc1]

Thanks @tavin for the note! This is in flux as we just changed this last week. The site repos are in https://github.com/executablebooks/myst-theme currently (in the themes folder). We moved them from being stand-alone to being built artifacts so that the install is faster when you run myst start and it downloads the first time.

This repo/folder is at least better place to look than the built files:
https://github.com/executablebooks/myst-theme/tree/main/themes/book

[sneakers-the-rat]

bump on this in the kindest, low-priority way possible - even just adding the links from eg. the book-theme readme to the myst-theme repo would be helpful :)