Add a README

... for the contributors' pleasure...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a GitHub ribbon

This ribbon should help hypothetical contributors find the correct place
to contribute.

Generated via https://codepo8.github.io/css-fork-on-github-ribbon/.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>