Add DEVELOPMENT.md to list common development commands

DEVELOPMENT.md

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+# Build
+
+1. Install `build` for example via `pacman -S python-build` on ArchLinux
+
+2. Afterwards run the command to generate pip packgases in `dist/`: `pyproject-build`
+
+This is also needed before development because the command generates `./src/nsupdate/_version.py`.
+
+# Run tests
+
+Tests need to run inside Docker because they depend on bind9 config.
+
+1. Build the docker image using: `docker build -t nsupdate scripts/docker/` once
+
+2. Then run tests via `docker run --dns 127.0.0.1 -v $PWD:/app nsupdate`
+
+# Lint
+
+Run pylint in error-only mode to check any problems: `pipenv run pylint src/nsupdate`
+
+# Test locally
+
+1. Create database using `pipenv run python ./manage.py migrate`
+
+2. Create a superuser with `pipenv run python ./manage.py createsuperuser`
+
+2. Run the server with `pipenv run python ./manage.py runserver`