add documentation for the use of labels
add documentation for the use of labels

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