gplugin/gplugin

Document clang-format in HACKING
develop
2020-02-15, Richard Laager
d286b22e30f0
Document clang-format in HACKING

This also renames HACKING to HACKING.md, as we are using Markdown
formatting in the file.
The following individuals have contributed to this project.
Ankit Vani <a@nevitus.org>
EionRobb <eion@robbmob.com>
Elliott Sales de Andrade <quantum.analyst@gmail.com>
Gary Kramlich <grim@reaperworld.com>
Jason Scherer <schereja@gmail.com>
John Bailey <rekkanoryo@rekkanoryo.org>
Mike Ruprecht <cmaiku@gmail.com>
Olaf Hering <olaf@aepfle.de>
Richard Laager <rlaager@pidgin.im>