doc: Add optional perl modules used during «make check» to README

README

@@ -77,6 +77,9 @@ To run the test suite («make check»):
 
   TimeDate perl module
   IO-String perl module
+  Test::Pod perl module (optional)
+  Test::Perl::Critic perl module (optional)
+  Perl::Critic::Utils perl module (optional)
 
   Define the environment variable DPKG_DEVEL_MODE to run the test suite
   in development mode, to include tests that might not be pertinent during