add a note about host's kernel to supported environments
@@ -20,9 +20,11 @@ Supported environments
----------------------
- Kubernetes: 1.20, 1.19, 1.18
-- Node OS: Linux with LVM2
+- Node OS: Linux with LVM2 (*1)
- Filesystems: ext4, xfs
+*1 The host's Linux Kernel must be v4.9 or later which supports `rmapbt` and `reflink`, if you use xfs filesystem with an official docker image.
+
Features
--------
