docs: Document new RO repo mirrors
docs: Document new RO repo mirrors

In the upstream discussion on creating a github mirror [1], it turned
out that there are some read-only mirrors of our repository. Lets
advertise them on our downloads page. But do it wisely and discourage
people in sending a pull requests on GitHub.

1: https://www.redhat.com/archives/libvir-list/2015-May/msg00775.html

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

nertpinx/libvirt,agx/libvirt,crobinso/libvirt,datto/libvirt,nertpinx/libvirt,nertpinx/libvirt,agx/libvirt,VenkatDatta/libvirt,eskultety/libvirt,eskultety/libvirt,agx/libvirt,jfehlig/libvirt,nertpinx/libvirt,crobinso/libvirt,jardasgit/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,datto/libvirt,crobinso/libvirt,VenkatDatta/libvirt,libvirt/libvirt,eskultety/libvirt,jardasgit/libvirt,jfehlig/libvirt,jfehlig/libvirt,andreabolognani/libvirt,zippy2/libvirt,libvirt/libvirt,jardasgit/libvirt,eskultety/libvirt,VenkatDatta/libvirt,zippy2/libvirt,agx/libvirt,fabianfreyer/libvirt,libvirt/libvirt,olafhering/libvirt,andreabolognani/libvirt,taget/libvirt,rlaager/libvirt,zippy2/libvirt,olafhering/libvirt,eskultety/libvirt,fabianfreyer/libvirt,zippy2/libvirt,datto/libvirt,jardasgit/libvirt,taget/libvirt,nertpinx/libvirt,rlaager/libvirt,datto/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,olafhering/libvirt,rlaager/libvirt,taget/libvirt,agx/libvirt,fabianfreyer/libvirt,taget/libvirt,taget/libvirt,olafhering/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,datto/libvirt,libvirt/libvirt,rlaager/libvirt,rlaager/libvirt,crobinso/libvirt,andreabolognani/libvirt