instuctions for the cmake building in Readme

[shimwell]

Just wanted to adds a few extra instructions to the readme to help users compile using cmake, I can make a PR for this

git clone git@github.com:open-radiation-sources/parametric-plasma-source.git
cd parametric-plasma-source
mkdir build
cd build
cmake .. -DOPENMC_DIR=~/openmc   # your path to the openmc repository
make
``

this will create a ```source_sampling.so``` file