add short Readme for benchmarks

Demos/benchmarks/Readme_benchmarks.rst

+To run a benchmark:
+
+10 iterations, python::
+
+  python nqueens.py -n 10
+
+
+10 iterations, cython::
+
+  python -c 'import nqueens;print(nqueens.test_n_queens(10))'
+

Demos/benchmarks/setup.py

@@ -9,5 +9,5 @@ setup(
   name = 'benchmarks',
   ext_modules = cythonize("*.py", language_level=3, annotate=True,
                           compiler_directives=directives,
-                          exclude="setup.py"),
+                          exclude=["setup.py","chaos.py"]),
 )