update readme again

explain how to get Debian kernel image

Signed-off-by: Bo YU <tsu.yubo@gmail.com>

README.md

@@ -13,11 +13,16 @@ sudo DOCKER_BUILDKIT=1 docker-compose build vf2
 sudo KERNEL_VERSION=vf2-v6.6.8-v1 docker-compose up vf2
 ```
 
-Please note, any modify need to be done with step first.
-
 Because there is no Debian kernel, which was missing some patch to boot vf2, so I have to maintain
 the kernel by myself.
 
+The kernel repo is [here](https://github.com/yuzibo/vf2-linux/tree/master)
+But I use different branch to generate kernel image which is under tags.
+Now it is based on [vf2-v6.6.8-dev](https://github.com/yuzibo/vf2-linux/tree/vf2-v6.6.8-dev) branch.
+
+
+Please note, any modify need to be done with step first.
+
 One image files will be generated into `image`:
 
 ```bash