add instructions for devs

Author: cbouy

DEVELOPMENT.md

@@ -0,0 +1,35 @@
+This is a short guide to setup a dev environment for mols2grid.
+
+1. Install conda or mamba
+2. Create a new environment. Python 3.7+ (prefer 3.8):
+   ```
+   conda env create --name mols2grid --file docs/environment.yml
+   ```
+3. Install all the package dependencies in editable mode:
+   ```
+   pip install -e .[dev]
+   ```
+
+To run tests locally:
+- Install Firefox (needed for UI testing)
+- Test your installation:
+  ```
+  pytest tests/
+  ```
+- You can select/skip the UI testing by specifying the `webdriver` mark in the pytest
+  command: `-m webdriver` to select UI tests only, or `-m "not webdriver"` to skip them.
+
+We use `black` and `isort` for formatting so either install the corresponding extension
+from your IDE or install the package with `pip install black isort`. The configuration
+is done inside the `pyproject.toml` file.
+
+Making a pull request will automatically run the tests and documentation build for you.
+Don't forget to update the `CHANGELOG.md` file with your changes.
+
+For versioning, you'll have to update both `package.json` and `mols2grid/_version.py`
+files.
+
+The build and deployment process is run automatically when making a release on
+GitHub.
+To make a prerelease, bump the versions accordingly (`X.Y.Z-rc1` format) and run
+the `build` GitHub Action manually.