made `def main` an explicit requirement

docsite/rst/developing_modules.rst

@@ -695,6 +695,7 @@ The following  checklist items are important guidelines for people who want to c
 * The return structure should be consistent, even if NA/None are used for keys normally returned under other options.
 * Are module actions idempotent? If not document in the descriptions or the notes.
 * Import module snippets `from ansible.module_utils.basic import *` at the bottom, conserves line numbers for debugging.
+* The module must have a `main` function that wraps the normal execution.
 * Call your :func:`main` from a conditional so that it would be possible to
   import them into unittests in the future example::