update README.md for kernel 5.11

Signed-off-by: Akihiro Suda <[email protected]>

Author: AkihiroSuda

README.md

@@ -1,6 +1,12 @@
 # [`fuse-overlayfs`](https://github.com/containers/fuse-overlayfs) snapshotter plugin for [containerd](https://containerd.io)
 
-Unlike `overlayfs`, `fuse-overlayfs` can be used as a non-root user without patching the kernel.
+Unlike `overlayfs`, `fuse-overlayfs` can be used as a non-root user on almost all recent distros.
+
+You do NOT need this `fuse-overlayfs` plugin on the following environments, because they support the real `overlayfs` for non-root users:
+- [kernel >= 5.11](https://github.com/torvalds/linux/commit/459c7c565ac36ba09ffbf24231147f408fde4203)
+- [Ubuntu kernel, since circa 2015](https://kernel.ubuntu.com/git/ubuntu/ubuntu-bionic.git/commit/fs/overlayfs?id=3b7da90f28fe1ed4b79ef2d994c81efbc58f1144)
+- [Debian 10 kernel](https://salsa.debian.org/kernel-team/linux/blob/283390e7feb21b47779b48e0c8eb0cc409d2c815/debian/patches/debian/overlayfs-permit-mounts-in-userns.patch)
+  - Debian 10 needs `sudo modprobe overlay permit_mounts_in_userns=1`. Future release of Debian with kernel >= 5.11 will not need this `modprobe` hack.
 
 ## Requirements
 * kernel >= 4.18