Add note about overlay not being production ready

Add a paragraph in cli.md mentioning that overlay is not a production
ready graphdriver.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
(cherry picked from commit 67cb748e26)

docs/sources/reference/commandline/cli.md

@@ -283,6 +283,12 @@ set `zfs.fsname` option as described in [Storage driver options](#storage-driver
 The `overlay` is a very fast union filesystem. It is now merged in the main
 Linux kernel as of [3.18.0](https://lkml.org/lkml/2014/10/26/137).
 Call `docker -d -s overlay` to use it.
+> **Note:**
+> As promising as `overlay` is, the feature is still quite young and should not
+> be used in production. Most notably, using `overlay` can cause excessive
+> inode consumption (especially as the number of images grows), as well as
+> being incompatible with the use of RPMs.
+
 > **Note:**
 > It is currently unsupported on `btrfs` or any Copy on Write filesystem
 > and should only be used over `ext4` partitions.