added note about building on latest gtkmm / Fedora 23

issue #6

README

@@ -41,3 +41,15 @@ Any valid preset banks that are found in these directories will be show in the
 drop-down bank selector on the GUI (standalone version only.)
 
 Plug-in versions can load presets by right-clicking on the GUI's background.
+
+
+Building on Fedora 23 / latest gtkmm
+------------------------------------
+
+The latest versions of gtkmm require C++11 support, but it is not enabled by
+default in current versions of GCC.
+
+If you encounter build errors relating to C++11 support, please configure
+the build like so:
+
+    ./configure CXXFLAGS='-std=c++11'