Explain developer actions/processes

[pp-mo]

It's not trivial to locally build (if you can't use cibuildwheel, as we can't here at MetOffice)
Nor :

• install for dev
• build docs
• make lockfiles
• run tests ?

Iris wraps these things nicely with nox, so you can replicate CI operations.
That's not so easy here (with tox), as the non-Python requirements are not fully controllable via conda/pip (notably: gcc, udunits2)
Leads to problems for developers/contributors (e.g. #432 #389).

So, we should explain these things better.