Suggestion: Add a "quickstart" section to README

[oseiskar]

Could be as simple as

mkdir build
cd build
cmake ..
make -j10
../bin/ytrace --scene ../tests/materials1/materials1.json
# output is written to bin/out.png

This would help prospective users to kick the tires of the library without browsing through the source or CMakeFiles.

I am very positively surprised that the above worked out-of-the-box on my machine with zero compiler warnings or OpenGL issues, and the output looked nice too. Great work!