Add basic README.md file for platform/graphics

Change-Id: I804e2b4aceffb94c551e7829da35254cdcf9fceb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2270108Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782978}

third_party/blink/renderer/platform/graphics/README.md

+<!---
+  The live version of this document can be viewed at:
+  https://chromium.googlesource.com/chromium/src/+/master/third_party/blink/renderer/platform/graphics/README.md
+-->
+
+# Platform graphics code
+
+This directory contains graphics support code with minimal external dependencies
+(e.g., no references to [`core`](../../core)). The main subdirectories are:
+
+- [`compositing`](compositing/README.md) -- Contains the implementation
+  of the "blink compositing algorithm".
+- [`darkmode`](darkmode/README.md) -- Dark mode neural network classifier.
+- `filters` -- Filter effects.
+- `gpu` -- GPU-accelerated support code.
+- [`paint`](paint/README.md) -- Contains the implementation of display lists and
+  display list-based painting.