message: Mention imgui in readme
diff: @@ -127,6 +127,7 @@ The viewer application and the examples statically link with the Visionaray-comm
  Visionaray currently uses the following third-party libraries:
  - [CmdLine](https://github.com/abolz/CmdLine) library to handle command line arguments in the viewer and example applications.
  +- [dear imgui](https://github.com/ocornut/imgui) library for GUI elements in the viewer and example applications.
  - [tinyply](https://github.com/ddiakopoulos/tinyply) library to load Stanford PLY models.
  