Add an annotation about html body element's 8px default margin to README.md

For additional explanation, add an annotation about HTML body element's
8px default margin to core/layout/README.md with the corresponding spec link.

Bug: None
Change-Id: I9cfe9acf005f5725dab4b99f7e874c9cdcda37db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1886139Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Joonghun Park <pjh0718@gmail.com>
Cr-Commit-Position: refs/heads/master@{#710625}

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

@@ -223,6 +223,10 @@ The final location of these within an 800x600 frame is as:
     container: (492, 8  300x200)
     relpos:    (662, 58 70x80)
 
+The 8px is the default margin of HTML body element
+per https://html.spec.whatwg.org/multipage/rendering.html#the-page,
+which is specified in ../html/resources/html.css.
+
 See the [diagram](resources/flipped-blocks-relpos.svg) for full detail on
 dimensions of the involved elements.