docs: Describe basic contributor workflow

DEVELOP.md

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+
+# How to contribute
+
+
+## Installation
+
+Checkout the latest master.
+
+    git clone https://github.com/florisla/stm32loader.git
+
+Install in editable mode with development tools (preferable in a virtual
+environment).
+
+    python -m venv .venv
+    .\.venv\bin\activate
+    pip uninstall stm32loader
+    pip install --editable .[dev]
+
+
+## Testing
+
+Run pytest.
+
+    pytest .
+
+
+## Linting
+
+Run flake8, pylint and black.
+
+    flake8 stm32loader
+    pylint stm32loader
+    black --check stm32loader 
+
+
+## Commit messages
+
+I try to follow the 'conventional commits' commit message style;
+see https://www.conventionalcommits.org/ .
+
+
+## Bump the version number
+
+    bumpversion --new-version 1.0.8-dev bogus-part
+
+
+## Tag a release
+
+First, bump the version number to a release version.
+Then create the git tag.
+
+    git tag -a "v1.0.9" -m "release: Tag version v1.0.9"
+
+Also push it to upstream.
+
+    git push origin v1.0.9