docs: Add a development guide

DEVELOPMENT.md

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+# Development
+
+Quickstart for functions-framework maintainers.
+
+## Environment setup
+
+Install [tox](https://pypi.org/p/tox), the test runner:
+
+```
+$ python -m pip install -U tox
+```
+
+## Running linting
+
+Linting can be run with:
+
+```
+$ python -m tox -e lint
+```
+
+## Running tests
+
+All tests can be run with:
+
+```
+$ python -m tox
+```
+
+Tests for the current Python version can be run with:
+
+```
+$ python -m tox -e py
+```
+
+Tests for a specific Python version can be run with:
+
+```
+$ python -m tox -e py3.12
+```
+
+A specific test file (e.g. `tests/test_cli.py`) can be run with:
+
+```
+$ python -m tox -e py -- tests/test_cli.py
+```
+
+A specific test in the file (e.g. `test_cli_no_arguements` in `tests/test_cli.py`) can be run with:
+
+```
+$ python -m tox -e py -- tests/test_cli.py::test_cli_no_arguments
+```
+
+## Releasing
+
+Releases are triggered via the [Release Please](https://github.com/apps/release-please) app, which in turn kicks off the [Release to PyPI](https://github.com/GoogleCloudPlatform/functions-framework-python/blob/main/.github/workflows/release.yml) workflow.