Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to data-diff
+
+Contributions are very welcome! We'll be happy to help you in the process.
+
+## What should I know before I get started?
+
+Go through the README and the documentation, and make sure that you understand how data-diff works. 
+
+## How to contribute?
+
+### Reporting bugs
+
+Please report the bug with as many details as you can.
+
+1. Include the exact command that you used. Make sure to run data-diff with the `-d` flag for debug output.
+2. Provide the entire output of the command. (stdout, logs, exception) 
+3. If possible, show us how we could reproduce the bug. i.e. how to set up an environment in which it occurs.
+
+(When pasting, always make sure to redact sensitive information, like passwords.)
+
+If data-diff returns incorrect results, i.e. false-positive or false-negative, please also include the original values.  
+
+Before you report a bug, make sure it doesn't already exist.
+
+See [issues](/datafold/data-diff/issues/).
+
+### Suggesting Enhancements
+
+We are always interested to hear about how we can make data-diff better!
+
+If you'd like us to support a new database, you should open an issue for it, if there isn't one already. If it already exists, make sure to vote for it with a :thumbsup:, to help us priortize it.
+
+The same goes for other technical requests, like missing features, or gaps in the documentation.
+
+See [issues](/datafold/data-diff/issues/).
+
+For questions, and non-technical discussions, see [discussions](/datafold/data-diff/discussions).
+
+### Contributing code
+
+#### Code style
+
+All code should be formatted with `black -l 120`.
+
+When in doubt, use the existing code as a guideline, or ask.
+
+#### Get started (setup)
+
+To get started, first clone the repository. For example `git clone https://github.com/datafold/data-diff`.
+
+Once inside, you can install the dependencies.
+
+- Option 1: Run `poetry install` to install them in a virtual env. You can then run data-diff using `poetry run data-diff ...` .
+
+- Option 2: Run `pip install -e .` to install them, and data-diff, in the global context.
+
+At the bare minimum, you need MySQL to run the tests.
+
+You can create a local MySQL instance using `docker-compose up mysql`. The URI for it will be `mysql://mysql:Password1@localhost/mysql`. If you're using a different server, make sure to update `TEST_MYSQL_CONN_STRING` in `tests/common.py`. For your convenience, we recommend creating `tests/local_settings.py`, and to override the value there.
+
+You can also run a few servers at once. For example `docker-compose up mysql postgres presto`.
+
+Make sure to update the appropriate `TEST_*_CONN_STRING`, so that it will be included in the tests.
+
+#### Run the tests
+
+You can run the tests with `unittest`.
+
+When running against multiple databases, the tests can take a long while. 
+
+To save time, we recommend running them with `unittest-parallel`.
+
+When debugging, we recommend using the `-f` flag, to stop on error. Also, use the `-k` flag to run only the individual test that you're trying to fix.
+
+#### Implementing a new database.
+
+New databases should be added as a new module in the `data-diff/databases/` folder.
+
+Make sure to update the `DATABASE_TYPES` dictionary in `tests/test_database_types.py`, so that it will be included in the tests.
+
+If possible, please also add the database setup to `docker-compose.yml`, so that we can run and test it for ourselves. If you do, also update the CI (`ci.yml`).