PyAlbany: Update the README.md to clarify the SWIG_EXECUTABLE option …

…and the mpi4py requirement

PyAlbany/README.md

@@ -6,7 +6,7 @@ PyAlbany is a Python wrapper for Albany.
 
 PyAlbany requires additional required dependencies:
 * A python executable (either Python 2 or 3),
-* Two python packages: numpy and mpi4py,
+* Two python packages: numpy and mpi4py (Important: mpi4py should be installed such that mpi4py is linked with the MPI library used during the compilation of Trilinos),
 * Swig 3.0.11 or higher (PyAlbany has been tested with swig 3.0.11 and 3.0.12).
 
 ### Configuration
@@ -63,6 +63,12 @@ CMake should be enabled to find the Python include path by itself. However, it h
 -D PYTHON_INCLUDE_PATH=${MY_PYTHON_DIR} \
 ```
 
+As said in the previous section on the configuration of Trilinos, the following command should be used to specify the python executable and the swig executable:
+```
+-D PYTHON_EXECUTABLE="/usr/bin/python3" \
+-D SWIG_EXECUTABLE="/usr/bin/swig" \
+```
+
 ### Testing
 
 PyAlbany can be tested using the following command from the Albany build directory: