[red-knot] Add mypy_primer usage documentation (#16679)

## Summary

Add documentation on how to run mypy_primer locally.

crates/red_knot/docs/mypy_primer.md

@@ -0,0 +1,61 @@
+# Running `mypy_primer`
+
+## Basics
+
+For now, we use our own [fork of mypy primer]. It can be run using `uvx --from "…" mypy_primer`. For example, to see the help message, run:
+
+```sh
+uvx --from "git+https://github.com/astral-sh/mypy_primer.git@add-red-knot-support" mypy_primer -h
+```
+
+Alternatively, you can install the forked version of `mypy_primer` using:
+
+```sh
+uv tool install "git+https://github.com/astral-sh/mypy_primer.git@add-red-knot-support"
+```
+
+and then run it using `uvx mypy_primer` or just `mypy_primer`, if your `PATH` is set up accordingly (see: [Tool executables]).
+
+## Showing the diagnostics diff between two Git revisions
+
+To show the diagnostics diff between two Git revisions (e.g. your feature branch and `main`), run:
+
+```sh
+mypy_primer \
+    --type-checker knot \
+    --old origin/main \
+    --new my/feature \
+    --debug \
+    --output concise \
+    --project-selector '/black$'
+```
+
+This will show the diagnostics diff for the `black` project between the `main` branch and your `my/feature` branch. To run the
+diff for all projects, you currently need to copy the project-selector regex from the CI pipeline in `.github/workflows/mypy_primer.yaml`.
+
+You can also take a look at the [full list of ecosystem projects]. Note that some of them might still need a `knot_paths` configuration
+option to work correctly.
+
+## Avoiding recompilation
+
+If you want to run `mypy_primer` repeatedly, e.g. for different projects, but for the same combination of `--old` and `--new`, you
+can use set the `MYPY_PRIMER_NO_REBUILD` environment variable to avoid recompilation of Red Knot:
+
+```sh
+MYPY_PRIMER_NO_REBUILD=1 mypy_primer …
+```
+
+## Running from a local copy of the repository
+
+If you are working on a local branch, you can use `mypy_primer`'s `--repo` option to specify the path to your local copy of the `ruff` repository.
+This allows `mypy_primer` to check out local branches:
+
+```sh
+mypy_primer --repo /path/to/ruff --old origin/main --new my/local-branch …
+```
+
+Note that you might need to clean up `/tmp/mypy_primer` in order for this to work correctly.
+
+[fork of mypy primer]: https://github.com/astral-sh/mypy_primer/tree/add-red-knot-support
+[full list of ecosystem projects]: https://github.com/astral-sh/mypy_primer/blob/add-red-knot-support/mypy_primer/projects.py
+[tool executables]: https://docs.astral.sh/uv/concepts/tools/#tool-executables