Add CHANGELOG.md

Author: mdboom

CHANGELOG.md

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+## Unreleased
+
+## v2.0.0
+
+Most of the work has moved from GitHub Actions `.yml` files to Python code in `workflow.py`.
+In the future, this will allow supporting more workflow engines beyond just GitHub Actions.
+
+**Migration note**: After running `python -m bench_runner install` to update your local files, but sure to add the new `workflow_bootstrap.py` file to your git repository.
+
+### New configuration
+
+Runners have a new configuration `use_cores` to control the number of CPU cores
+used to build CPython. By default, this will use all available cores, but some
+Cloud VMs require using fewer.