Add a link to "Using ICU BiDi in LayoutNG" to README.md

NOTRY=True
TBR=yosin@chromium.org

Change-Id: If8f1a4632b151e26372a54351ba3d232b0d3bdad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547745Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828932}

third_party/blink/renderer/core/layout/ng/inline/README.md

@@ -356,6 +356,8 @@ In a bird's‐eye view, it consists of two parts:
 
    This is part of the Line Box Construction phase above.
 
+Initial design doc: [Using ICU BiDi in LayoutNG](https://docs.google.com/document/d/182H1Sj_FCEHcl6eC69J4KcIc5m3ohSzgo297KYB0S_c/edit?usp=sharing)
+
 ### Interface for Editing ###
 
 [NGOffsetMapping] provides functions for converting between offsets in the text