Update docs for requirements_dev

[odscjames]

https://opendataservices.gitbook.io/developer-docs/code/code-python#pinned-dependencies

Sometimes we put -r requirements.in in https://github.com/OpenDataServices/cove/blob/master/requirements_dev.in#L1- in this case can we update commands for how to do this?

When you pip freeze req_dev you have to remove that line by hand from req_dev.txt - is this right?

Noticed this when doing open-contracting/kingfisher-collect#92

Note #66 - not all repos have an update_script - what do we want to do with that?