Add LAMMPS shared library instructions for CMake

doc/src/Python_shlib.txt

@@ -9,6 +9,8 @@ Documentation"_ld - "LAMMPS Commands"_lc :c
 
 Build LAMMPS as a shared library :h3
 
+Build LAMMPS as a shared library using make :h4
+
 Instructions on how to build LAMMPS as a shared library are given on
 the "Build_basics"_Build_basics.html doc page.  A shared library is
 one that is dynamically loadable, which is what Python requires to
@@ -32,4 +34,40 @@ extra libraries must also be shared libraries.  If the LAMMPS
 shared-library build fails with an error complaining about this, see
 the "Build_basics"_Build_basics.html doc page.
 
-TODO: Also include CMake info on this
+Build LAMMPS as a shared library using CMake :h4
+
+When using CMake the following two options are necessary to generate the LAMMPS
+shared library:
+
+-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!) :pre
+
+What this does is create a liblammps.so which contains the majority of LAMMPS
+code. The generated lmp binary also dynamically links to this library. This
+means that either this liblammps.so file has to be in the same directory, a system
+library path (e.g. /usr/lib64/) or in the LD_LIBRARY_PATH.
+
+If you want to use the shared library with Python the recommended way is to create a virtualenv and use it as
+CMAKE_INSTALL_PREFIX.
+
+# create virtualenv
+virtualenv --python=$(which python3) myenv3
+source myenv3/bin/activate :pre
+
+# build library
+mkdir build
+cd build
+cmake -D PKG_PYTHON=on -D BUILD_LIB=on -D BUILD_SHARED_LIBS=on -D CMAKE_INSTALL_PREFIX=$VIRTUAL_ENV ../cmake
+make -j 4 :pre
+
+# install into prefix
+make install :pre
+
+This will also install the Python module into your virtualenv. Since virtualenv
+doesn't change your LD_LIBRARY_PATH, you still need to add its lib64 folder to
+it, which contains the installed liblammps.so.
+
+export LD_LIBRARY_PATH=$VIRTUAL_ENV/lib64:$LD_LIBRARY_PATH :pre
+
+Starting Python outside (!) of your build directory, but with the virtualenv
+enabled and with the LD_LIBRARY_PATH set gives you access to LAMMPS via Python.