Clarify what's required in order to use Centos images with overlayfs

centos/content.md

@@ -20,6 +20,10 @@ Additionally, images with minor version tags that correspond to install media ar
 
 For example: `docker pull centos:5.11` or `docker pull centos:6.6`
 
+## Overlayfs and yum
+
+Recent Docker versions support the [overlayfs](https://docs.docker.com/engine/userguide/storagedriver/overlayfs-driver/) backend, which is enabled by default on most distros supporting it from Docker 1.13 onwards. On Centos 6 and 7, **that backend requires yum-plugin-ovl to be installed and enabled**; while it is installed by default in recent centos images, make it sure you retain the `plugins=1` option in `/etc/yum.conf` if you update that file; otherwise, you may encounter errors related to rpmdb checksum failure - see [Docker ticket 10180](https://github.com/docker/docker/issues/10180) for more details.
+
 # Package documentation
 
 By default, the CentOS containers are built using yum's `nodocs` option, which helps reduce the size of the image. If you install a package and discover files missing, please comment out the line `tsflags=nodocs` in `/etc/yum.conf` and reinstall your package.