added minimal README

cmake/README

+cmake-buildsystem
+-----------------
+
+To use the cmake build system instead of the make-driven one, do:
+```
+cmake /path/to/lammps/source/cmake
+```
+(please note the cmake directory as the very end)
+
+To enable package, e.g. GPU do
+```
+cmake /path/to/lammps/source/cmake -DENABLE_GPU=ON
+```
+
+cmake has many many options, do get an overview use the curses-based cmake interface, ccmake:
+```
+ccmake /path/to/lammps/source/cmake
+```
+(Don't forget to press "g" for generate once you are done with configuring)