Add few notes about Python implementation and building the Cython

kernels

heat-equation/python/README.md

+# Python implementation
+
+The Python implementation utilizes NumPy and mpi4py for computation and
+communication, as well as matplotlib for outputting images. The main program
+is implemented in [heat.py](heat.py), and the whole program can be executed as
+
+    mpirun -np xxx python heat.py
+
+
+## Using Cython kernel
+
+By default, the code uses pure NumPy for evaluating the time evolution
+step. In addition, a Cython version for the main computational kernels
+is provided in [evolve.pyx](evolve.pyx). The Cython kernels can be built
+into Python extension in current directory with
+
+    python setup.py build_ext --inplace
+
+After building the Cython kernels main program can be executed as previously.