Add paragraph on how libcellml could be used with the help of external tools.
Add paragraph on how libcellml could be used with the help of external tools.

libcellml/buildbot-testing,cellml/libcellml,nickerso/libcellml,MichaelClerx/libcellml,hsorby/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,hsorby/libcellml,MichaelClerx/libcellml,nickerso/libcellml,cellml/libcellml,libcellml/buildbot-testing,cellml/libcellml,hsorby/libcellml,nickerso/libcellml,MichaelClerx/libcellml