Add a naming conventions category to blink/public/README.md

As discussed on platform-architecture-dev ML[1], this CL
adds the guideline to public/README.md file for claryfing
the naming conventions in public/web/, platform/, and common/

[1] https://groups.google.com/a/chromium.org/g/platform-architecture-dev/c/qrZLqP4ndJE/m/uiBiljN9BwAJ

Bug: None
Change-Id: I51eceae18b16a282b61a65f7ff4d0f16c566c114
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429982Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#810522}

third_party/blink/public/README.md

@@ -81,6 +81,13 @@ details of Blink, and consumers of the API should not rely upon these types.
 Similarly, the API uses STL types outside of the INSIDE_BLINK preprocessor
 macro, which is for the convenience of the consumer.
 
+Naming Conventions
+------------------
+
+The public/common directory doesn't need to use 'Web' prefix for their classes,
+structs, and enums. But, public/platform/ and public/web/ directories keep using
+'Web' prefix for the internal consistency.
+
 Contact Information
 -------------------