--- /dev/null
+# Gauging changes
+
+A lot of the time, your change will affect formatting and/or performance. Quantifying
+these changes is hard, so we have tooling to help make it easier.
+
+It's recommended you evaluate the quantifiable changes your _Black_ formatting
+modification causes before submitting a PR. Think about if the change seems disruptive
+enough to cause frustration to projects that are already "black formatted".
+
+## black-primer
+
+`black-primer` is a tool built for CI (and humans) to have _Black_ `--check` a number of
+Git accessible projects in parallel. (configured in `primer.json`) _(A PR will be
+accepted to add Mercurial support.)_
+
+### Run flow
+
+- Ensure we have a `black` + `git` in PATH
+- Load projects from `primer.json`
+- Run projects in parallel with `--worker` workers (defaults to CPU count / 2)
+ - Checkout projects
+ - Run black and record result
+ - Clean up repository checkout _(can optionally be disabled via `--keep`)_
+- Display results summary to screen
+- Default to cleaning up `--work-dir` (which defaults to tempfile schemantics)
+- Return
+ - 0 for successful run
+ - \< 0 for environment / internal error
+ - \> 0 for each project with an error
+
+### Speed up runs 🏎
+
+If you're running locally yourself to test black on lots of code try:
+
+- Using `-k` / `--keep` + `-w` / `--work-dir` so you don't have to re-checkout the repo
+ each run
+
+### CLI arguments
+
+```{program-output} black-primer --help
+
+```