* Use environment variable for format to build
* Use Makefile instead of docker-compose.yml
This allows us to build different formats more easily, and use phd and
doc-base from parallel checkouts, which is much easier for issues that
span the repositories.
Add links to the new contribution guidelines and to the translation repositories.
Change the file's extension to .md.
Make minor formatting changes to the document.
Co-authored-by: haszi <haszika80@gmail.com>
