Add small readme file to examples directory

src/examples/README.md

+SWE/src/examples
+================
+
+Contains example programs that use SWE.
+
++ **swe_simple.cpp** A "simple" example that only runs on one core. Instead of the CPU it can also use the GPU for wave propagation.
++ **swe_mpi.cpp** Similar to the example above, but it can run on more the one node using MPI. If used with CUDA it requires one GPU per MPI task.
++ **swe_opengl.cpp** An example program that uses the OpenGL visualization.
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