Document adding a new dependency and updating conda-forge package

[jfrost-mo]

What problem does your feature request solve?

Currently the developer documentation doesn't cover how to add a new dependency, and deal with the conda-forge package update needed. This prevents me being the single point of failure.

Describe the solution you'd like

I've written some documentation in Issue #763. This documentation should be edited, then added to the sphinx documentation. Specifcially the relevent comments are:

• Where in the source repository needs updating
• How to add dependencies to conda-forge package, and update that

Describe alternatives you've considered

None.