update instructions in Python doc section

.. code-block:: bash

   make foo mode=shlib

   make foo mode=shared

where foo is the machine target name, such as mpi or serial.

This should create the file liblammps_foo.so in the src directory, as

.. code-block:: bash

   -D BUILD_LIB=on            # enable building LAMMPS as a library

   -D BUILD_SHARED_LIBS=on    # enable building of LAMMPS shared library (both options are needed!)

What this does is create a liblammps.so which contains the majority of LAMMPS

   # build library

   mkdir build

   cd build

   cmake -D PKG_PYTHON=on -D BUILD_LIB=on -D BUILD_SHARED_LIBS=on -D CMAKE_INSTALL_PREFIX=$VIRTUAL_ENV ../cmake

   cmake -D PKG_PYTHON=on -D BUILD_SHARED_LIBS=on -D CMAKE_INSTALL_PREFIX=$VIRTUAL_ENV ../cmake

   make -j 4

   # install into prefix