First cut at an FAQ for iperf3.

Most of the text contributed by @bltierney.

docs/faq.rst

@@ -0,0 +1,75 @@
+iperf3 FAQ
+==========
+
+What is the history of iperf3, and what is the difference between iperf2 and iperf3?
+  iperf2 was orphaned in the late 2000s at version 2.0.5, despite some
+  known bugs and issues. After spending some time trying to fix
+  iperf2's problems, ESnet decided by 2010 that a new, simpler tool
+  was needed, and began development of iperf3. The goal was make the
+  tool as simple as possible, so others could contribute to the code
+  base. For this reason, it was decided to make the tool single
+  threaded, and not worry about backwards compatibility with
+  iperf2. Many of the feature requests for iperf3 came from the
+  perfSONAR project (http://www.perfsonar.net).
+ 
+  Then in 2014, Bob (Robert) McMahon from Broadcom restarted
+  development of iperf2 (See
+  https://sourceforge.net/projects/iperf2/). He fixed many of the
+  problems with iperf2, and added a number of new features similar to
+  iperf3. iperf2.0.8, released in 2015, made iperf2 a useful tool.
+ 
+  As of this writing (2017), both iperf2 and iperf3 are being actively
+  (although independently) developed.  We recommend being familiar with
+  both tools, and use whichever tool’s features best match your needs.
+ 
+  A feature comparison of iperf2, iperf3, and nuttcp is available at:
+  https://fasterdata.es.net/performance-testing/network-troubleshooting-tools/throughput-tool-comparision/
+ 
+iperf3 parallel stream performance is much less than iperf2. Why?
+  iperf3 is single threaded, and iperf2 is multi-threaded. We
+  recommend using iperf2 for parallel streams.
+ 
+I’m trying to use iperf3 on Windows, but having trouble. What should I do?
+  iperf3 is not officially supported on Windows, but iperf2 is. We
+  recommend you use iperf2.
+
+  Some people are using Cygwin to run iperf3 in Windows, but not all
+  options will work.  Some community-provided binaries of iperf3 for
+  Windows exist.
+ 
+I'm seeing quite a bit of unexpected UDP loss. Why?
+  First, confirm you are using iperf 3.1.5 or higher. There was an
+  issue with the default UDP send size that was fixed in
+  3.1.5. Second, try adding the flag ``-w1M`` to increase the socket
+  buffer sizes. That seems to make a big difference on some hosts.
+ 
+iperf3 UDP does not seem to work at bandwidths less than 1Mbps. Why?
+  The pacing algorithm used by iperf3 does not allow for send rates
+  less than 1Mbps.
+ 
+What congestion control algorithms are supported?
+  On Linux, run this command to see the available congestion control
+  algorithms (note that some algorithms are packaged as kernel
+  modules, which must be loaded before they can be used)::
+    
+    /sbin/sysctl net.ipv4.tcp_available_congestion_control
+ 
+I’m using the ``--logfile`` option. How do I see file output in real time?
+  Use the ``--forceflush`` flag.
+ 
+Why can’t I run a UDP client with no server?
+  This is potentially dangerous, and an attacker could use this for a
+  denial of service attack.  We don't want iperf3 to be an attack tool.
+
+I have a question regarding iperf3...what's the best way to get help?
+  Searching on the Internet is a good first step.
+  http://stackoverflow.com/ has a number of iperf3-related questions
+  and answers, but a simple query into your favorite search engine can
+  also yield some results.
+
+  There is a mailing list nominally used for iperf3 development,
+  iperf-dev@googlegroups.com.
+
+  We discourage the use of the iperf3 issue tracker on GitHub for
+  support questions.  Actual bug reports, enhancement requests, or
+  pull requests are encouraged, however.

docs/index.rst

@@ -53,6 +53,7 @@ Contents
    building
    invoking
    dev
+   faq
 
 Indices and tables
 ------------------