Add a reference to continuing iperf2 development in the README.md file.

Suggested by @beau-williamson in #27.

README.md

@@ -100,6 +100,13 @@ submit an issue.  Please use one of the mailing lists for that.
 Changes from iperf 2.x
 ----------------------
 
+(Note that iperf2 is no longer being developed by its original
+maintainers.  However, beginning in 2014, another developer began
+fixing bugs and enhancing functionality, and generating releases of
+iperf2.  Both projects (as of late 2017) are currently being developed
+actively, but independently.  The continuing iperf2 development
+project can be found at https://sourceforge.net/projects/iperf2/.)
+
 New options:
 
     -V, --verbose             more detailed output than before