plans: add the "PLANS" directory

plans: add the "PLANS" directory

Individual files or sub-directories about future and on-going
development plans in Automake will be added in follow-up commits.

This new set of documents is meant to help ensure a more controlled
and smooth development and evolution for Automake, in several ways.

 - Having the plans clearly spelled out should will avoid messy
   roadmaps with no clear way forward or with muddy or ill-defined
   aims or purposes; a trap this is too easy to fall into.

 - Keeping planned changes cooking and re-hashed for a while should
   ensure rough edges are smoothed up, transitions are planned in a
   proper way (hopefully avoiding debacles like the AM_MKDIR_PROG_P
   deprecation and the AM_CONFIG_HEADER too-abrupt removal), and
   "power users" have more chances of getting informed in due time,
   thus having all the time to prepare for the changes or raise
   objections against them.

 - Having the plans clearly stated and registered in a "centralized"
   location should make it more difficult to them to slip through
   the cracks, getting forgotten or (worse) only half-implemented.

 - Even for discussions and plans registered on the Bug Tracker
   as well, a corresponding entry in the PLANS directory can help
   in keeping main ideas summarized, and consensus and/or objections
   registered and easily compared.

Motivation:
<http://blog.flameeyes.eu/2013/01/autotools-mythbuster-automake-pains>
Not a flatting picture for us (and maybe a little too harsh), but
basically true and even spot-on in some regards.

* PLANS/README: New.
* Makefile.am (EXTRA_DIST): Distribute the whole PLANS directory.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>