Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+If you want to contribute please do it like this:
+1. Create a fork of the project
+2. Perform the contribution on your fork
+3. Create a pull request
+
+## Testing
+- All tests must be created in the `test` directory.
+- The tests should be located in a package of the same name as the code it tests.
+- The name of the file should be the same as the file of the code it tests, with a `tests_` prefix.
+
+For example, the tests for `html_to_django/formatters/attr_formatter.py` are located in `tests/formatters/tests_attr_formatter.py`.
+If you make changes to the code you need to make sure all the tests are still working. Use this command to run the tests:
+```bash
+python -m unittest discover tests --verbose
+```
+
+## Stylechecks
+Follow established python coding guidelines. To check your code you can use these commands:
+```bash
+mypy --strict html_to_django
+pycodestyle --max-line-length 120 html_to_django
+```
+
+Change `html_to_django` to `tests` to also check your tests.