Update CONTRIBUTING.md

Rename README.tests.md to CONTRIBUTING.md and add information about
running tests and pre-commit checks.

CONTRIBUTING.md

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+# Contributing
+
+## Pre-commit
+
+We use [pre-commit] to run a series of [linters] on the code in this
+repository. These checks will be run whenever you submit a pull request. In
+order to avoid delays in getting your pull request reviewed, you will want to
+enable these same checks as part of your local development workflow:
+
+1. Install the [pre-commit] tool.
+1. From the root of this repository, run:
+
+    ```
+    pre-commit install
+    ```
+
+This will modify `.git/hooks/pre-commit` in your local repository to run the
+pre-commit checks each time you make a commit. Among other things, these
+linters will ensure consistent formatting of Python code in the repository and
+alert on a variety of syntax errors.
+
+[pre-commit]: https://pre-commit.com/
+[linters]: https://en.wikipedia.org/wiki/Lint_(software)
+
+## Unit tests
+
+### Install prerequisites
+
+Install the dependencies from `test-requirements.txt`:
+
+```
+pip install -r test-requirements.txt
+```
+
+### Run the tests
+
+To run the unit tests, from the top level of the repository run:
+
+```
+pytest
+```
+
+To generate HTML coverage reports:
+
+```
+pytest --cov-report=html
+```
+
+Then open `htmlcov/index.html` in your browser.