Add extra-cxxflags example

configure

@@ -22,6 +22,8 @@ Building options:
     --cpu-flags=<CPUFLAGS>      CPU defining/optimizing flags to be used. [configure autodetected]
     --extra-cflags=<CFLAGS>     Extra C flags passed to C compilation. []
     --extra-cxxflags=<CXXFLAGS> Extra C++ flags passed to C++ compilation. []
+                                Example: --extra-cxxflags="-DMY_RX_MESSAGE_BUFFER_SIZE=\(32\) \
+                                -DMY_RF24_DATARATE=\(RF24_2MBPS\)"
     --extra-ldflags=<LDFLAGS>   Extra C flags passed to linking. []
     --c_compiler=<CC>           C compiler. [arm-linux-gnueabihf-gcc][gcc]
     --cxx_compiler=<CXX>        C++ compiler. [arm-linux-gnueabihf-g++][g++]
@@ -32,7 +34,7 @@ Building options:
 
 Installation options:
     --prefix=<PREFIX>           Installation prefix path. [/usr/local]
-    --gateway-dir=<DIR>         Gateway files installation directory. [PREFIX/bin]       
+    --gateway-dir=<DIR>         Gateway files installation directory. [PREFIX/bin]
 
 MySensors options:
     --my-debug=[enable|disable] Enables or disables MySensors core debugging. [enable]