Document our plan for calculating expenditure/commitments

Created by: rjw57

Add a small document outlining how we intend to calculate expenditure and commitments going forward. This PR attempts to crystalise our discussions while they are still fresh.

Recall that GitHub will render the Jupyter notebook for you. E.g.: https://github.com/rjw57/pidash-ucamstaffoncosts/blob/document-future-direction/doc/salary-calculations.ipynb