release: generate a stub for the release announcement

release: generate a stub for the release announcement

It's much better than having to write it my hand each time; after all,
most of it is either boilerplate or a cope of NEWS entries.

* Makefile.am (determine_release_type): Also set the shell variable
'$announcement_type' appropriately.
(print-release-type): Print the value of this new variable as well.
(announcement): New phony target, generate a files with the same name.
The recipe uses the shell variable '$announcement_type'.
(CLEANFILES): Clean it.
(PACKAGE_MAILINGLIST): New make macro, used when generating the
'announcement' file.
* HACKING: Explain how to take advantage of the new convenience
target.
* .gitignore: Ignore the 'announcement' file.

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