Added readme

This closes issue #5.

README.md

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+# QtOSG: combining Qt and OSG in a thread-safe manner
+
+This repository contains a widget based on `QOpenGLWidget` that is able
+to wrap a viewer from the [OpenSceneGraph](http://www.openscenegraph.org) library.
+This makes it possible to combine both toolkits in a thread-safe manner.
+
+Moreover, the widget demonstrates several interaction mechanisms:
+
+1. Rectangular selection processing
+2. Pick handling
+3. Node file writing (for debug purposes)
+
+# Requirements
+
+* A recent C++ compiler with support for C++11
+* `CMake` (minimum version 2.8.11)
+* `Qt5`
+* `OpenSceneGraph`
+
+# Building QtOSG
+
+    $ git clone https://github.com/Submanifold/QtOSG
+    $ mkdir build
+    $ cd build
+    $ cmake ../
+    $ make
+
+Additional build options can be configured by issuing `ccmake .` in the
+build directory. In particular, you can toggle the following options:
+
+* `WITH_PICK_HANDLER`: toggle to compile with/without support for point
+  picking
+* `WITH_SELECTION_PROCESSING`: toggle to compile with/without support
+  for rectangular selections
+
+# Using QtOSG
+
+The demo application merely demonstrates basic usage of the widget.
+Don't expect too much! Start the application by issuing `./qtosg` in the
+build directory.
+
+The following key bindings are active:
+
+* `d`: writes the current scene graph to `/tmp/sceneGraph.osg`
+* `h`: resets the view to home 
+* `s`: toggles selection processing (if compiled); if active, hold left
+  mouse button down to draw a selection rectangle; selected objects will
+  be shown on the console
+
+# How to contribute
+
+Please see the [contribution guidelines](CONTRIBUTING.md) for more
+information.
+
+# Licence
+
+`QtOSG` uses the MIT licence. Pleas see the file
+[LICENSE.md](LICENSE.md) in the main directory of the repository for
+more information.