Added CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing to Causal Pyro
+
+## Development
+
+Please follow our established coding style including variable names, module imports, and function definitions. The Causal Pyro codebase follows the [PEP8 style guide](https://www.python.org/dev/peps/pep-0008/) (which you can check with `make lint`) and follows [`isort`](https://github.com/timothycrosley/isort) import order (which you can enforce with `make format`). 
+
+## Dev Setup
+To install dev dependencies for Causal Pyro, run the following command.
+```sh
+pip install -e .[test]
+```
+
+## Testing
+
+Before submitting a pull request, please autoformat code and ensure that unit tests pass locally
+```sh
+make lint              # linting
+make format            # runs black and isort
+make tests             # linting and unit tests
+```
+
+## Submitting
+
+For relevant design questions to consider, see past [design documents](https://github.com/pyro-ppl/pyro/wiki/Design-Docs).
+
+For larger changes, please open an issue for discussion before submitting a pull request.
+
+In your PR, please include:
+- Changes made
+- Links to related issues/PRs
+- Tests
+- Dependencies
+
+For speculative changes meant for early-stage review, include `[WIP]` in the PR's title. (One of the maintainers will add the `WIP` tag.)
+
+## Code of conduct
+This project follows [GitHub community guidelines](https://help.github.com/en/github/site-policy/github-community-guidelines) and [Pyro code of conduct](https://github.com/pyro-ppl/pyro/blob/dev/CODE_OF_CONDUCT.md).

Makefile

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+lint: FORCE
+	scripts/lint.sh
+
+format:
+	black .
+	isort .
+
+tests: lint FORCE
+	pytest -v tests
+
+FORCE: