Add contrib/ folder to downloads.openwrt.org

[Paul Spooren]

Hi all,

due to the recent discussion of SELinux enabled ImageBuilders created 
by buildbots, I'd like to suggest a slightly different approach:

We could offer a contrib/ folder on downloads.openwrt.org (d.o.o) which 
contains such builds, created independently from the buildbots.

The folder is not meant to contain community builds for single devices 
(like frequently found in forums) nor special package selections (use 
e.g. Chef[1] for that). Only full builds should be provided to test 
possible feautres of the future: GCC10, SELinux, Full Language Support 
(NLS), more?

I've been playing around with GitLab, GitHub and SourceHut CIs and can 
setup something.

As a goal, more people can test things which are normally hidden behind 
the barrier of self compiling. Additionally developers have a better 
coverage of default disabled features.

These builds cost computer power, so they should happen at best effort 
and not with the 24h demand we have for snapshots and packages.

Please share your thoughts and objections.

Best,
Paul

[1]: https://chef.libremesh.org/