Add dev setup notes

DEVELOPMENT.md

@@ -0,0 +1,23 @@
+# Development
+
+## Setup
+Clone the repo locally, and create a Python virtual env (recommend using [pew](https://github.com/berdario/pew)):
+```bash
+pew new preserve
+```
+
+Inside the repo (beside `setup.py`) and inside the virtual env, install as an editable python package, using the `dev` extras with:
+```bash
+pip install -e ./[dev]
+```
+
+## Testing
+### During development
+Run `pytest` within venv to run test suite and flake8 (linting+style) checks.
+
+To get a coverage report, run `pytest --cov preserve`. Run `coverage html` afterward or `pytest --cov preserve --cov-report html` to generate a html report with line-specific coverage data.
+
+Assuming `pew` was used for the venv, running `pew wipeenv` will uninstall all packages to allow the pip install to be run from scratch.
+
+### Before release
+Run `tox` within venv to check the tests for all supported python versions, and check package installation all works. Use `tox -r` to force recreation of every environment.