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>
Upstream-commit: 67cb748e26c3b6979983493f307a38b59e291642
Component: engine

components/engine/docs/sources/reference/commandline/cli.md

@@ -292,6 +292,12 @@ 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.