chore: add test readme doc

Author: ascorbic

test/e2e/README.md

@@ -0,0 +1,34 @@
+# Next.js e2e test suite
+
+The tests in this directory are taken from the Next.js monorepo.
+[See the licence](https://github.com/vercel/next.js/blob/canary/license.md). The files in `tests` run unmodofied. The
+ones in `modified-tests` either have fixes to run correctly, or have tests disabled. The tests in `disabled-tests` are
+current disabled because of either an incompatibility or bug.
+
+The tools in `next-test-lib` are based on the equivalent tools in the Next.js monorepo. The original utilities test the
+Next.js code, whereas here they are used to test the Netlify runtime. For this reason only the "deploy" mode is enabled,
+and it builds using the runtime packages in this monorepo and deploys using the Netlify CLI. The e2e tests then run
+against the deployed site.
+
+The script that runs these is in
+[.github/workflows/e2e-next.yml](https://github.com/netlify/next-runtime/blob/main/.github/workflows/e2e-next.yml). It
+runs all the tests in parallel using a matrix, with each running in a separate job. The tests are run against the
+version of the Next Runtime that is currently in the monorepo, and the version of `next` that is in the monorepo's
+dependencies.
+
+To run the tests locally, the npm script `test:next` can be used, which runs the default tests. The script
+`test:next:all` runs all the tests, including the ones in `disabled-tests` and the other tests skipped by Jest. The
+script `test:next:disabled` runs only the disabled tests. Unlike in CI, these are run sequentially, so will take a long
+time to run, so you may want to just run a single test suite. To run a single suite run
+`npm run test:next:all -- the-test-name-here`, e.g. `npm run test:next:all -- streaming-ssr`. The name is matched as a
+pattern against the path of the test file and can be a partial match.
+
+When they are run, the tests generate the sites in a temporary directory, and then deploy them to Netlify. The e2e tests
+are then run against these. This can be overridden by setting the env var `SITE_URL`, which will be used instead of
+deploying the test site. This only makes sense when running a single test suite, because each test suite runs against a
+different site.
+
+The tmp directory with each test site is deleted after the tests have been run. If you want to manually build or deploy
+the test sites, you can run the test command, and then you can kill the test process after it has generated the site.
+The location of the tmp directory is printed in the console. Alternatively, set the env var `NEXT_TEST_SKIP_CLEANUP` and
+the site directories will all be retained.