ml purge in to introduction table

docs.it4i/software/lmod.md

@@ -19,6 +19,7 @@ Below you will find more details and examples.
 | ml                       | module list                                                      |
 | ml GCC/6.2.0-2.27        | module load GCC/6.2.0-2.27                                       |
 | ml -GCC/6.2.0-2.27       | module unload GCC/6.2.0-2.27                                     |
+| ml purge                 | module unload all modules                                        |
 | ml av                    | module avail                                                     |
 | ml show GCC/6.2.0-2.27   | module show GCC                                                  |
 | ml spider                | gcc searches (case-insensitive) for gcc in all available modules |