[doc] Add developer documentation

Since switching away from do.yp to scripts/fabricate, the developer
experience has changed drastically. This commit starts adding more
formal documentation on how a developer interacts with the code
base.

Resolves #2230
Partially fixes #2238

DEVELOPMENT.md

@@ -0,0 +1,123 @@
+Working with the `coco` code base
+=================================
+
+## Before you start
+
+To work with the `coco` source code, you need quite a few tools in very specific versions.
+We provide a Conda environment file `env.yaml` and recommend you use it to setup your development environment.
+It is regularly tested on Linux, Windows and macOS and is what is used in CI.
+
+Following these steps should setup a fresh `coco` environment:
+
+1. Create or update a new conda environment with all the requirements by running
+
+   ```sh
+   conda env create -f env.yaml
+   ``` 
+   or if you've already setup a `coco` environment
+   ```sh
+   conda env update -f env.yaml --prune
+   ``` 
+
+1. Activate environment
+   ```sh
+   conda activate coco
+   ```
+
+You now have all required dependencies to work on the code base.
+If you notice that something is missing, please let us know by opening an [issue](https://github.com/numbbo/coco/issues/new/choose).
+
+## Working with the Experimental Code
+
+Before any of the bindings can be built and after every change to the core files (in `code-experiments/src/`), you need to run `scripts/fabricate`.
+The job of `fabricate` is to bundle the C files and place them in all the right places so that the language specific build tools can find them when building the respective bindings.
+`fabricate` also updates the binding metadata to include version information and any other changes that need to be made from one build to the next.
+It is harmless to run `fabricate` too often, but forgetting to run it means you might be using stale sources.
+
+### Running unit tests
+
+Change to the unit tests directory
+```sh
+cd code-experiments/test/unit-test
+```
+
+Rerun `fabricate` if not already done
+```sh
+python ../../../scripts/fabricate
+```
+
+Build tests using [`cmake`](https://cmake.org/cmake/help/latest/manual/cmake.1.html)
+```sh
+cmake -B build
+cmake --build build
+```
+
+Run tests using [`ctest`](https://cmake.org/cmake/help/latest/manual/ctest.1.html)
+```sh
+ctest --test-dir build     
+```
+
+### Regression tests
+
+Note that I'm not sure the regression tests are really useful at the moment. But here goes:
+
+Change to the regression tests directory
+```sh
+cd code-experiments/test/regression-test
+```
+
+The regression tests require the `cocoex` Python package. Install it first
+```sh
+python -m pip uninstall -y cocoex
+python -m pip install ../../build/python/  
+```
+
+Now run the regression tests
+
+```sh
+python test_suites.py   
+```
+and 
+```sh
+python test_logger.py
+```
+
+### Integration tests
+
+Still need to fix those up.
+
+### `cocoex` Python package
+
+Change to the `cocoex` Python package directory
+```sh
+cd code-experiments/build/python
+```
+
+Install the package from source
+```sh
+pip uninstall -y cocoex
+pip install .
+```
+
+Install and run `pytest`
+```sh
+pip install pytest
+python -m pytest test
+```
+
+## Working with the Postprocessing Code
+
+Before you begin, always run `python scripts/fabricate` to update any auto-generated files.
+Then, you can install the `cocopp` package using pip:
+
+```sh
+python -m pip uninstall -y cocopp
+python -m pip install code-postprocessing
+```
+
+If you are working on `cocopp`, you can use an editable install and Python will pick up any changes you make without having to reinstall:
+
+```sh
+python -m pip uninstall -y cocopp
+python -m pip install --editable code-postprocessing
+```