Add a workflow to run the test-suite on Ubuntu 18.04 and Mac OSX ...

Using Github actions gives much more control to contributors.
Ultimately, we'd like to add Windows support too,
but currently the testsuite is too tied to POSIX.

Add a sponsorship button

use xml comments in issue template ...

- so that github editor users don't have to delete those paragraphs

add issue template ...

Ask for normal information when reporting issues to avoid roundtrips.