Set qt >=5.6 as a "global" dependency.

i.e. not just required by the netinstall module.

README.md

@@ -14,8 +14,7 @@
 Main:
 * Compiler with C++11 support: GCC >= 4.9.0 or Clang >= 3.5.1
 * CMake >= 2.8.12
-* Qt >= 5.3
- * Note: this changes if you compile the netinstall module, see list below.
+* Qt >= 5.6
 * yaml-cpp >= 0.5.1
 * Python >= 3.3
 * Boost.Python >= 1.55.0
@@ -36,8 +35,6 @@ Modules:
 * unpackfs:
  * squashfs-tools
  * rsync
-* netinstall:
- * Qt >= 5.6
 
 ### Building