Add renderer/core/dom/events/README.md

TBR=tkent@chromium.org

Change-Id: I7ac71397fab24f23ee8e860f8cf476a1ad4cd332
Reviewed-on: https://chromium-review.googlesource.com/c/1322179Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605976}

third_party/blink/renderer/core/dom/events/README.md

+# DOM Events
+
+[Rendered](https://chromium.googlesource.com/chromium/src/+/master/third_party/blink/renderer/core/dom/events/README.md)
+
+Author: hayato@chromium.org
+
+The `renderer/core/dom/events` directory contains the implementation of [DOM Events].
+
+[DOM Events]: https://dom.spec.whatwg.org/#events
+
+Please avoid to put any kind of specific event's implementation in this directory, such as [UI Events].
+
+[UI Events]: https://w3c.github.io/uievents/