Add some docs on how to run the tests.

tests/visual-regression/README.md

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+# Visual Regression Tests in WordPress Core
+
+These tests make use of Jest and Puppeteer, with a setup very similar to that of the e2e tests, together with [jest-image-snapshot](https://github.com/americanexpress/jest-image-snapshot) for generating the visual diffs.
+
+## How to Run the Tests Locally
+
+1. Check out trunk.
+2. Run `npm run test:visual` to generate some base snapshots.
+3. Check out the feature branch to be tested.
+4. Run `npm run test:visual` again. If any tests fail, the diff images can be found in `tests/visual-regression/specs/__image_snapshots__/__diff_output__`.
+