Standardise developer doc location in repos

[endorama]

This repository contains a CONTRIBUTING.md file that is automatically used by GitHub for all repositories in the organisation.

This is great as it provides consistency, but it also means is not possible to include repository specific documentation.

I would like to propose adding a conventional path where to add a developer doc to each repo, so can be mentioned from the contributing file.

In the wild I've seen used HACKING.md and DEVELOPMENT.md, we could also use docs/developer.md.

Thoughts? (If this is accepted I'll work in updating the contributing file and adding the relevant file to each repo)