Add CI notes to CONTRIBUTING.md

CONTRIBUTING.md

@@ -48,6 +48,18 @@ the platform, and provide a unique non-trivial challenge to modern
 machine learning algorithms. Feel free to submit these environments with a
 PR explaining the nature of the environment and task.
 
-## Style Guide
+## Continuous Integration (CI)
 
-When performing changes to the codebase, please ensure that all python code is reformatted using the [black](https://github.com/ambv/black) formatter. For C#, we will soon be requirements for style and formatting.
+We run CircleCI on all PRs; all tests must be passing before the PR is merged.
+
+Several static checks are run on the codebase using the [pre-commit framework](https://pre-commit.com/) during CI. To execute the same checks locally, install `pre-commit` and run `pre-commit run --all-files`. Some hooks (for example, `black`) will output the corrected version of the code; others (like `mypy`) may require more effort to fix.
+
+### Code style
+All python code should be formatted with [`black`](https://github.com/ambv/black). Style and formatting for C# may be enforced later.
+
+### Python type annotations
+We use [`mypy`](http://mypy-lang.org/) to perform static type checking on python code. Currently not all code is annotated but we will increase coverage over time. If you are adding or refactoring code, please
+1. Add type annotations to the new or refactored code.
+2. Make sure that code calling or called by the modified code also has type annotations.
+
+The [type hint cheat sheet](https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html) provides a good introduction to adding type hints.