Adding notes on PyGRIFFIN configuration strategies to the README.

# PyGriffin

## Configuring PyGriffin

There are a number to indicate location of the Griffin executable to PyGriffin.

  1. Pass the path to the Griffin executable to the constructor of the PyGriffin

     class.

  2. Set the `GRIFFIN_EXEC` environment variable in the shell environment

     PyGriffin is called from.

  3. Set the value of `griffin_path` in a .pygriffin.rc configuration file a the

     top of the PyGriffin repository.

These will be preferentially used by PyGriffin in the order they are provided by

the user and the user's compute environment.