README: mention that we're unstable
README: mention that we're unstable

Resolves: #152

Closes: #153
Approved by: ignatenkobrain

hughsie/libhif,edynox/libdnf,rpm-software-management/libdnf,Conan-Kudo/libhif,edynox/libdnf,rpm-software-management/libhif,edynox/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf,Conan-Kudo/libhif,rpm-software-management/libhif,edynox/libdnf,rpm-software-management/libhif,rpm-software-management/libhif,Conan-Kudo/libhif,Conan-Kudo/libhif,hughsie/libhif,rpm-software-management/libdnf,hughsie/libhif,hughsie/libhif