Update README

Author: liuyangzhuan

PYTHON/README

@@ -20,7 +20,8 @@ There is one example in the directory.
 1. pddrive.py: 
    This example will generate a random SPD matrix, perform the factorization and computes the log-determinant (and compare it with log-determinant from numpy.linalg.slogdet)    
    To run the code, type:
-      python pddrive.py -c 2 -r 1 
+      export SUPERLU_PYTHON_LIB_PATH=[PATH to the superlu_dist_python library]
+      mpirun -n 2 python pddrive.py -c 2 -r 1 
    (The above use 2 MPIs.)
    Note that if a 64-bit integer interface is needed (assuming -DXSDK_INDEX_SIZE=64 has been used to compile the code), set INT64 = 1 in pddrive.py