Add Bluetooth class diagram documentation references to README files.

content/browser/bluetooth/README.md

Index: content/browser/bluetooth/README.md
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+Bluetooth in Content
+====================
+
+`content/*/bluetooth` implements [Web Bluetooth][WB] using the
+`device/bluetooth` code module.
+
+[WB]: https://webbluetoothcg.github.io/web-bluetooth/
+
+Testing
+--------------------------------------------------------------------------------
+
+Bluetooth layout tests in `third_party/WebKit/LayoutTests/bluetooth/` rely on
+fake Bluetooth implementation classes constructed in
+`content/shell/browser/layout_test/layout_test_bluetooth_adapter_provider`.
+These tests span JavaScript binding to the `device/bluetooth` API layer.
+
+
+Design Documents
+--------------------------------------------------------------------------------
+
+See: [Class Diagram of Web Bluetooth through Bluetooth Android][Class]
+
+[Class]: https://sites.google.com/a/chromium.org/dev/developers/design-documents/bluetooth-design-docs/web-bluetooth-through-bluetooth-android-class-diagram
+