contrib: new, a directory for non-mainstream functionalities

contrib: new, a directory for non-mainstream functionalities

This new 'contrib' hierarchy will be a good place were to move
implementation/support for obsolescent features we are not yet
ready to remove completely, or were to leave experimental or
third-party features to cook before their eventual inclusion in
the automake core.

* contrib: New directory.
* contrib/README: New file.
* contrib/Makefile.am: New file.
(EXTRA_DIST): Distribute README.