File Release Notes and Changelog

* Add a TODO list.

* Fix a potential memory leak in memcache_server_add(): this function
  might have unwittingly allocated memory in TopMemoryContext.

* Make the "port" argument to memcache_server_add() optional. If not
  specified, it defaults to the libmemcache default (11211).

* Add a "dist" target to the Makefile.