Create Developer-targeted Documentation

It would be helpful for new developers and community contributors to have some documentation specific to how to set up your development environment and how to get started developing code for AtoM. I find it can be a bit difficult to get started writing code for AtoM if you're not a seasoned dev, I think having some of the setup documented would help reduce the barrier to entry to get community contributors on-board.

Some things I think would be useful for this documentation:

Specific setup instructions for your operating system (Linux/MacOS/Windows/WSL).
I use WSL on Windows and could include specific setup instructions for this.
A colleague I work with, @yenaing-oo, uses an ARM-based Mac and could include specific setup instructions for that architecture.
How to debug the app in Docker depending on your IDE
I use VS Code and could provide a launch config for this, for example
Quick reference for project commands (like running php-cs-fixer, running the unit tests, running Webpack, etc.)
Any extra scripts used to facilitate writing code, like [this logging setup](https://gist.github.com/melaniekung/c7479af4f3b3b6c911e58a5779897152)
And a question open for discussion is, where should this documentation go? This documentation feels specific to this repository, so it could be stored somewhere here. For example, maybe a DEVELOPERS.md file would be appropriate.

The issue proposes a DEVELOPERS.md file in this repository, but leaves the final location open. Start by reviewing the repository's existing documentation structure, then cover the listed operating-system setup, Docker debugging, IDE guidance, project commands, and contributor scripts. Done means new contributors can follow the documented setup and development workflow.

Written by the indexing model from the issue text.