add usage instructions to README

README.md

@@ -20,11 +20,30 @@ Once you run it, you can use SlicerIGT (module SlicerIGTIF) to connect and recei
 transform called "SpaceMouse".
 
 
+## Buttons ##
+
+The two buttons of the SpaceMouse are currently mapped as follows:
+* Button 0 (left button): Reset translation
+* Button 1 (right button): Reset rotation
+
+
+## Usage in 3D Slicer ##
+
+3D Slicer needs the OpenIGTLink extension to make this work.
+
+1. Open 3D Slicer
+2. Start spnav-igt
+3. In Slicer, go to "OpenIGTLinkIF" extension and create a new connection.
+4. Set the port to the port of spnav-igt (default: 18944).
+5. Hit the "Active" checkbox.
+6. Go to "Data" extension, "Transform hierarchy" tab.
+7. Drag "Camera" (or the camera you want to manipulate) under "SpaceMouse".
+
 ## TODOs ##
 
 * [x] CMake build with FetchContent (auto-install OpenIGTLink)
 * [x] Rotations (so far, only translations are supported)
 * [ ] Flexible configuration via JSON
 * [x] System-wide installation
-* [ ] Use button events somehow
+* [x] Use button events
 * [x] Command line arguments