Add a bit of documentation about device scanner.

README.md

@@ -70,3 +70,29 @@ back to a default backend suitable for the current platform.
 
 Calling KPMcore functions before the library is initialized will
 result in undefined behavior.
+
+### Devices [FIXME: WIP]
+
+After the backend is initialized you can scan for available devices.
+If you want only devices from the loaded backend you can call
+
+```
+QList<Device*> devices = backend->scanDevices( excludeReadOnly );
+```
+
+where bool option `excludeReadOnly` specified whether to exclude
+read only devices.
+
+#### KPMcore device scanner
+
+Alternatively, you can use KPMcore device scanner which also finds
+LVM Volume Groups.
+
+```
+DeviceScanner *deviceScanner = new DeviceScanner(this, operationStack()))
+deviceScanner->start();
+```
+
+The `deviceScanner` scans for the devices in a background thread. After
+scanning is complete `DeviceScanner::finished()` signal will be emitted.
+Then the devices can accessed using `operationStack().previewDevices()`.