Try to finish off a first draft of CONTRIBUTING.md.

.github/CONTRIBUTING.md

@@ -7,5 +7,63 @@ and other sorts of interactions with the iperf3 project.  These are
 guidelines and not hard rules, and it is intended that common sense
 and good judgement will prevail.
 
+## Support
 
+iperf3 is officially supported on Linux (various distributions),
+FreeBSD, and macOS.  Support may be provided on a best-effort basis to
+other UNIX-like platforms.  We cannot provide support for building
+and/or running iperf3 on Windows, iOS, or Android.
 
+Before asking for help, please use your favorite search engine or
+questions site (such as Stack Overflow) to see if your question might
+have been asked (and maybe even answered) before.
+http://fasterdata.es.net/ has some information on the use of various
+bandwidth measurement tools, including iperf3.  The iperf3
+documentation Web site at http://software.es.net/iperf/ contains
+various bits of helpful information, including an FAQ.
+
+We specifically discourage the use of the issue tracker on the iperf3
+GitHub project page for asking questions.  Questions posted in the
+form of issues may go unanswered.  Please use a questions site
+such as Stack Overflow to ask questions of the community or
+alternatively use the iperf3 mailing list at
+iperf-dev@googlegroups.com (posting requires joining the list).
+
+## Code
+
+If you have improvements or bugfixes to make to iperf3, we'd love to
+hear from you.  We prefer changes to be submitted in the form of pull
+requests on GitHub, although we can (probably) accept simple patches
+as well.  If in doubt, ask.
+
+Before making any submission to the iperf3 project (whether it be code
+or documentation), we urge you to consult the iperf3 license, in
+particular the section quoted below:
+
+```You are under no obligation whatsoever to provide any bug fixes, patches, or
+upgrades to the features, functionality or performance of the source code
+("Enhancements") to anyone; however, if you choose to make your Enhancements
+available either publicly, or directly to Lawrence Berkeley National
+Laboratory, without imposing a separate written license agreement for such
+Enhancements, then you hereby grant the following license: a non-exclusive,
+royalty-free perpetual license to install, use, modify, prepare derivative
+works, incorporate into other computer software, distribute, and sublicense
+such enhancements or derivative works thereof, in binary and source code form.
+```
+
+If you're considering changes that will have an architectural impact,
+we strongly encourage discussing them with the iperf3 maintainers
+before doing a significant amount of work on the code.  We might be
+able to provide some guidance.  Also, we're more likely to accept a
+submission if if it doesn't involve rewriting large sections of the
+code.  Even if you're going to fork the code and maintain your own
+changes privately (which you're perfectly welcome to do) we might able
+to give you guidance so that future iperf3 changes won't conflict with
+your work.
+
+## Conduct
+
+We expect that iperf3 interactions via the issue tracker, mailing
+lists, and so forth will be conducted civilly.  Language that is
+deemed appropriate or abusive may be removed, and we reserve the right
+to ban users from accessing the project for repeated offense.