bug#19913: 25.0.50; Add target version(s) information to etc/CONTRIBUTE

[Eric Abrahamsen]

A bit of information that should be in etc/CONTRIBUTE, but isn't:

When contributing patches to Emacs proper, which versions should be
targeted? How should one get them as "compile-clean" as possible? At the
moment randomsample.de seems to be down, but I recall that the code is
automatically built against 24.1, 24.2, HEAD, and a couple of XEmacs
flavors. I also recall having a hard time finding git branches that
corresponded exactly to the buildbot versions. I'm also so far totally
unable to build XEmacs at all, but that's a separate problem.

So, under the "Supplemental information for Emacs Developers" heading, I
propose another heading that goes something like this:

** Backwards Compatibility

New Emacs code should be compatible with the following older versions:

- [LIST OLDER VERSIONS HERE]

Git branches tracking the above Emacs versions are in the official Git
repository, with branch names that look like "back_compat/*" [NOT TRUE,
I'M MAKING THIS UP, BUT I THINK IT'S A GOOD IDEA. WHERE DO THE
BUILDBOT'S TEST VERSIONS COME FROM, ANYWAY?]. XEmacs can be
obtained from http://www.xemacs.org/.

For larger patches, please take the time to apply your patch to these
different versions, and ensure that Emacs builds without error.

Note that the Emacs codebase is automatically built every X DAYS,
against each of the above-mentioned versions. Should any patch you
contribute cause an error in one of these version builds, you'll receive
an automatic notification email from the buildbot.



Anyway, I think something like that would be helpful.