README: clarify cmake package requirements

CMake 3.5.0 or later is required.  For Ubuntu 14.04, that has a
different package name.

Signed-off-by: Rick Altherr <kc8apf@kc8apf.net>
Signed-off-by: Tim 'mithro' Ansell <mithro@mithis.com>

README.md

@@ -10,7 +10,8 @@ Pull submodules:
     git submodule update --init --recursive
 
 Install CMake and build the C++ tools:
-    sudo apt-get install cmake3
+    sudo apt-get install cmake # version 3.5.0 or later required,
+                               # for Ubuntu Trusty pkg is called cmake3
     mkdir build
     pushd build
     cmake ..