Update the README

[dbarrow257]

Update the README to describe the updated installation instructions:

MaCh3_DUNE/README.md

Lines 21 to 35 in 91f3137

	Now setup some dependencies and then actually build MaCh3_DUNE
	~~~~~~~~~~~~~~~
	$ source setup.sh # !! Here you need to make sure that ROOTSYS and Cuda libraries are also set !!
	$ source setup_dune_env.sh
	$ mkdir build;
	$ cd build
	~~~~~~~~~~~~~~~
	Optional flags are described briefly below, options are shown grouped by square brackets and delimited by vertical lines. Default is on the left.
	~~~~~~~~~~~~~~
	$ cmake .. -DCPU_ONLY=[OFF|ON] -DUSE_PROB3=[OFF|ON] -DSINGLE_THREAD_ONLY=[OFF|ON] -DCUDA_SAMPLES=<path_to_cuda>/CentOS/samples
	$ make
	~~~~~~~~~~~~~~