Add readme for wpt_automation directory

Change-Id: I52d65971d0c049e7c7bc8c86a69303d60b26fa0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1908718Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Navid Zolghadr <nzolghadr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714316}

third_party/blink/web_tests/external/wpt_automation/README

+This folder is for Chrome automation specific only. If your manual test
+can be automated using test_driver APIs exposed in wpt documented in
+https://web-platform-tests.org/writing-tests/testdriver.html
+use those APIs and drop the "manual" suffix from the name of the test.
+
+If your test can only be automated with Chrome only testing APIs
+such as eventSender then leave the "manual" in the name of the test
+and add a mirror automation script in this directory. You need to also
+make sure your test subdirectory is whitelisted under
+resources/testharnessreport.js.
+
+If you add an automation script here please also file an issue to capture
+the follow up work with the owners in that space to make that testing API
+cross browser.