Enhance project template documentation

[davidB]

README is will become too long with all the details, rules. So provide the longer documentation (as html/pdf page) with more details about how to create a template. Template creation should stay simple and the documentation should not afraid creator ;-).

[ ] use mdbook
[ ] start with doc in README
[ ] add link in README
[ ] publish on gh-pages
[ ] update link in Cargo.toml
[ ] add badges

• after publish
• after publish