Add bit more documentation in WebKit/common/README.md

Bug: 740070
Change-Id: I1b0983d8f4e0c49466edf176bc4a1c7f73105539
Reviewed-on: https://chromium-review.googlesource.com/599252
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491948}

third_party/WebKit/common/README.md

 # Blink 'common' directory
 
-This directory contains the common Web Platform stuff that needs to be shared by renderer-side and browser-side code.
+This directory contains the common Web Platform stuff that needs to be shared
+by renderer-side and browser-side code.
 
-Things that live in third_party/WebKit/Source and browser-side code (e.g. content/browser for the time being) can both depend on this directory, while anything in this directory should NOT depend on them.
+Things that live in `third_party/WebKit/Source`, or other renderer-side code
+and browser-side code (e.g. `//content` and `//chrome`) can depend on this
+directory, while anything in this directory should **NOT** depend on them.
+(See `DEPS` and `BUILD.gn` files for more details)
 
-Unlike other directories in WebKit, code in this directory cannot use Blink specific types like WTF, but should use common chromium types.
+Code in this directory would normally use `blink` namespace.
+
+Unlike other directories in WebKit, code in this directory should:
+
+* Use Chromium's common types (e.g. //base ones) rather than Blink's ones
+  (e.g. WTF types)
+
+* Use underscore_separated_file_names.cc style rather than CamelCase.cpp.
+
+* Follow [Chromium's common coding style guide](https://chromium.googlesource.com/chromium/src/+/master/styleguide/c++/c++.md)
+
+* Use full-path from src/ for includes (e.g. `third_party/WebKit/common/foo.h` rather than `common/foo.h`)