rpm: doc: Remove bundled SPEC and update README

Packages targeting various distributions are now available thanks to the
Open Bulid Service. The SPECS have been moved to a separate repo under

https://github.com/cloud-hypervisor/obs-packaging

which documents how to include pre-compiled binaires in a supported
distribution. Besides the Cloud Hypervisor itself, accompanying pieces
like UEFI bins are available, too.

Any contributions to the packaging can be also accepted in
the aforementioned repo. Further integration with the OBS like PR
validation is to be targeted there, too, as far as it can get. As the
packaging topic has moved in a separate repo, the bundled SPEC became
obsolete is suggested for removal.

Fixes: #4257

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
Co-authored-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

README.md

@@ -9,6 +9,7 @@
   - [Install prerequisites](#install-prerequisites)
   - [Clone and build](#clone-and-build)
     - [Containerized builds and tests](#containerized-builds-and-tests)
+- [Use Prebuilt Binaries](#use-prebuilt-binaries)
   - [Run](#run)
     - [Cloud image](#cloud-image)
     - [Custom kernel and disk image](#custom-kernel-and-disk-image)
@@ -152,6 +153,15 @@ $ ./scripts/dev_cli.sh tests --unit
 Run the `./scripts/dev_cli.sh --help` command to view all the supported
 development script commands and their related options.
 
+## Use Prebuilt Binaries
+
+Cloud Hypervisor packages targeting some popular Linux distributions are available
+thanks to the [Open Build Service](https://build.opensuse.org). The
+[OBS README](https://github.com/cloud-hypervisor/obs-packaging) explains how to
+enable the repository in a supported Linux distribution and install Cloud Hypervisor
+and accompanying packages. Please report any packaging issues in the
+[obs-packaging](https://github.com/cloud-hypervisor/obs-packaging) repository.
+
 ## Run
 
 You can run a guest VM by either using an existing cloud image or booting into