Documentation: Update core/dom/README.md, adding a link to "Incremental Shadow DDM"

TBR=kochi

Bug: 742715
Change-Id: I1bc4e37d5d1e707276f38bc11dfae2fe93c2d260
Reviewed-on: https://chromium-review.googlesource.com/727389Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510019}

third_party/WebKit/Source/core/dom/README.md

@@ -498,6 +498,8 @@ If we are ignoring the effect of `<slot>`, we can have the following simple defi
 
 TODO(hayato): Explain.
 
+In the meantime, please see [Incremental Shadow DOM](https://docs.google.com/document/d/1R9J8CVaSub_nbaVQwwm3NjCoZye4feJ7ft7tVe5QerM/edit?usp=sharing).
+
 # FlatTreeTraversal
 
 TODO(hayato): Explain.