Summarize platform sensor support in device/generic_sensor/README.md

device/generic_sensor/README.md

 # Sensors
 
 `device/generic_sensor` contains the platform-specific parts of the Sensor APIs
 implementation.
 
 Sensors Mojo interfaces are defined in the `public/interfaces` subdirectory.
 
 The JS bindings are implemented in `third_party/WebKit/Source/modules/sensor`.
 
 
+## Platform Support
+
+Support for the SensorTypes defined by the Mojo interface is summarized in this
+table. An empty cell indicates that the sensor type is not supported on that
+platform.
+
+| SensorType           | Android                   | Linux          | macOS              | Windows                                   |
+| -------------------- | ------------------------- | -------------- | ------------------ | ----------------------------------------- |
+| AMBIENT_LIGHT        | TYPE_LIGHT                | in_illuminance | AppleLMUController | SENSOR_TYPE_AMBIENT_LIGHT                 |
+| PROXIMITY            |                           |                |                    |                                           |
+| ACCELEROMETER        | TYPE_ACCELEROMETER        | in_accel       |                    | SENSOR_TYPE_ACCELEROMETER_3D              |
+| LINEAR_ACCELEROMETER | TYPE_LINEAR_ACCELEROMETER |                |                    |                                           |
+| GYROSCOPE            | TYPE_GYROSCOPE            | in_anglvel     |                    | SENSOR_TYPE_GYROMETER_3D                  |
+| MAGNETOMETER         | TYPE_MAGNETIC_FIELD       | in_magn        |                    | SENSOR_TYPE_COMPASS_3D                    |
+| PRESSURE             |                           |                |                    |                                           |
+| ABSOLUTE_ORIENTATION | TYPE_ROTATION_VECTOR      |                |                    | SENSOR_TYPE_AGGREGATED_DEVICE_ORIENTATION |
+
+### Android
+
+Sensors are implemented by passing through values provided by the
+[Sensor](https://developer.android.com/reference/android/hardware/Sensor.html)
+class. The values in the "Android" column of the table above correspond to the
+integer constants from the android.hardware.Sensor used to provide data for a
+SensorType.
+
+### Linux (and Chrome OS)
+
+Sensors are implemented by reading values from the IIO subsystem. The values in
+the "Linux" column of the table above are the prefix of the sysfs files Chrome
+searches for to provide data for a SensorType.
+
+### macOS
+
+On this platform there is limited support for sensors. The AMBIENT_LIGHT sensor
+type is provided by interpreting the value that can be read from the LMU.
+
+### Windows
+
+Sensors are implemented by passing through values provided by the
+[Sensor API](https://msdn.microsoft.com/en-us/library/windows/desktop/dd318953(v=vs.85).aspx).
+The values in the "Windows" column of the table above correspond to the names of
+the sensor type GUIDs used to provide data for a SensorType.
+
 ## Testing
 
 Sensors platform unit tests are located in the current directory and its
 subdirectories.
 
 The sensors unit tests file for Android is
 `android/junit/src/org/chromium/device/sensors/PlatformSensorAndProviderTest.java`.
 
 Sensors browser tests are located in `content/test/data/generic_sensor`.
 
 
 ## Design Documents
 
 Please refer to the [design documentation](https://docs.google.com/document/d/1Ml65ZdW5AgIsZTszk4mD_ohr40pcrdVFOIf0ZtWxDv0)
 for more details.