Add WPG installation instruction

[JunCEEE]

As mentioned in #7 (comment)

An instruction to install WPG is needed.

I'd write somewhere in the documentation:
SimEx-Lite depends on WPG (github.com/Samoyl/WPG). Please download and build WPG as per their documentation. Then, you have to make the WPG library "visible" to SimEx-Lite. To this end, do one of the following:

$PYTHONPATH: If you have the environment variable $PYTHONPATH set, copy the WPG/ directory into any directory listed in $PYTHONPATH.
conda: If you installed SimEx-Lite in a conda environment, activate that environment and copy the WPG/ directory to $CONDA_PREFIX/lib/python3.x/site-packages/ (x is your python minor version).
pip: If you installed SimEx-Lite with pip install, copy the WPG directory into $HOME/.local/lib/python3.x/site-packages
Test that WPG can be imported by running:

$> python -c 'import WPG.Beamline'