Changes to Broxygen master script package index

- Now only lists packages as those directories in the script hierarchy
  that contain an __load__.bro file.
- Script packages (dirs with a __load__.bro file), can now include a
  README (in reST format) that will automatically be appended under
  the link to a specific package in the master package index.

doc/bin/group_index_generator.py

@@ -49,6 +49,7 @@ with open(group_list, 'r') as f_group_list:
                 if not os.path.exists(os.path.dirname(group_file)):
                     os.makedirs(os.path.dirname(group_file))
                 with open(group_file, 'w') as f_group_file:
+                    f_group_file.write(":orphan:\n\n")
                     title = "Package Index: %s\n" % os.path.dirname(group)
                     f_group_file.write(title);
                     for n in range(len(title)):