updated ltemplify.py docs

doc/doc_ltemplify.md

@@ -82,6 +82,7 @@ to the molecule of interest and customize the output.
 |-ignore-bond-types  | Ignore the 2nd column in the "Bonds" section of the LAMMPS data file, and create a "Bond List" section in the resulting MOLTEMPLATE LT file which omits the bond types.  (This is useful when using external force fields, such as OPLSAA.)| |
 |-ignore-masses | Ignore all masses in the data file.  Omit from output file.  (This is useful when using external force fields, such as OPLSAA.)|
 |-prepend-atom-type *STR* | prepend the string from the *STR* argument to the beginning of all atom type names. |
+|-preamble *STR* | Print the string *STR* at the beginning of the file generated by ltemplify.py.  (Example: *'import "oplsaa.lt"'*)  This argument can be used multiple times if you want to prepend multiple strings at the beginning of the file. |
 
 Examples showing typical argument usage are
 [included below](#-Examples).