Create CONTRIBUTING.md

CONTRIBUTING.md

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+Contributing
+===
+
+To install ZhuSuan in an "editable" or "develop" mode, run
+```
+pip install -e .
+```
+in the main directory. This installation is removable by
+```
+python setup.py develop --uninstall
+```
+Additional dependencies for developments can be installed by
+```
+pip install ".[dev]"
+```
+
+#### Tests
+
+This command will run automatic tests in the main directory
+```
+python -m unittest discover -v
+```
+
+##### Test Coverage
+After running tests, to ensure test coverage over the developments, run
+```
+coverage report --include="zhusuan/*"
+```
+
+##### PEP8 Code Style Check
+
+We follow PEP8 python code style. To check, in the main directory, run::
+```
+pep8 .
+```
+
+#### Docs
+
+Docs are written under the `docs/` directory as RestructuredText (`.rst`)
+files. `index.rst` is the main page. A Tutorial on RestructuredText can be
+found [here](https://pythonhosted.org/an_example_pypi_project/sphinx.html).
+
+API References are automatically generated by
+[Sphinx](http://www.sphinx-doc.org/en/stable/) according to the outlines under 
+`doc/api/` and should be modified when any code changes.
+
+To compile docs into webpages, Run
+```
+make html
+```
+under the `docs/` directory. The generated webpages are in `docs/_build` and
+can be viewed with browsers.