Add CONTRIBUTING.md (testcontainers#695)

CONTRIBUTING.md

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+# Contributing
+
+[Create an issue](https://github.com/testcontainers/testcontainers-node/issues) if you find any bugs.
+
+[Create a pull request](https://github.com/testcontainers/testcontainers-node/pulls) if you wish to fix an issue or provide an enhancement. Please be sure to:
+* Discuss with the authors via an issue or discussion prior to doing anything big.
+* Follow the style, structure and naming conventions of the rest of the project.
+* Make commits atomic and easy to merge.
+* Run the Git hooks when making commits to ensure the code is linted and correctly formatted.
+* Verify all tests are passing with `npm test`.
+
+## Documentation
+
+The documentation is a static site built with [MkDocs](https://www.mkdocs.org/) and the [Material for MkDocs](https://squidfunk.github.io/mkdocs-material/) theme. In addition, we use a [custom plugin](https://github.com/rnorth/mkdocs-codeinclude-plugin) for inclusion of code snippets. We publish our documentation using Netlify.
+
+### Previewing rendered content
+
+#### Using Docker
+
+The root of the project contains a `docker-compose.yml` file. Simply run `docker-compose up` and then access the docs at [http://localhost:8000](http://localhost:8000).
+
+#### Using Python
+
+* Ensure that you have Python 3.8.0 or higher.
+* Set up a virtualenv and run `pip install -r requirements.txt` in the `testcontainers-node` root directory.
+* Once Python dependencies have been installed, run `mkdocs serve` to start a local auto-updating MkDocs server.
+
+#### PR Preview deployments
+
+Documentation for pull requests will automatically be published by Netlify as 'deploy previews'. These deployment previews can be accessed via the `deploy/netlify` check that appears for each pull request.