new Github options for bug reporting and feature requests

Hi all - Axel, with help from Richard Berger and Christoph Junghans,
has enabled various options on the LAMMPS Github server, which now
allow anyone to do the following:

submit bug reports:

submit new-feature requests:

submit Git pull requests with code you've
  written to either fix bugs or implement new features:

Axel is his usual diligent self in responding quickly to posted
issues/ideas, and to vetting code rapidly to get it into the main
LAMMPS distribution. So we encourage folks to use this capability.
If a further heads-up or discussion is warranted, you can of course
also post Qs or notices to the mail list of what you've done or found.