README.md: encourage to use librepo

Document `--use-librepo` in the README and encourage its usage
in the default example. It should be fine for centos and this
way we hopefully get more exposure and bugreport (if there are
issues).

README.md

@@ -57,6 +57,7 @@ sudo podman run \
     quay.io/centos-bootc/bootc-image-builder:latest \
     --type qcow2 \
     --local \
+	--use-librepo=True \
     quay.io/centos-bootc/centos-bootc:stream9
 ```
 
@@ -153,6 +154,7 @@ Global Flags:
 | --target-arch     | [Target arch](#-target-architecture) to build                                                             |       ❌      |
 | --log-level       | Change log level (debug, info, error)                                                                     |     `error`   |
 | -v,--verbose      | Switch output/progress to verbose mode (implies --log-level=info)                                         |     `false`   |
+| --use-librepo     | Download rpms using librepo (faster and more robust)                                                      |     `false`   |
 
 The `--type` parameter can be given multiple times and multiple
 outputs will be produced. Note that comma or space separating the