osbuilder: add description for how to use DISTRO variable
Fixes:
@@ -95,6 +95,9 @@ filesystem components to generate an initrd.
3. When generating an image, the initrd is extracted to obtain the base rootfs for
the image.
+CentOS is the default distro for building the rootfs, to use a different one, you can set `DISTRO=<your_distro>`.
+For example `make USE_DOCKER=true DISTRO=ubuntu rootfs` will make Ubuntu rootfs using Docker.
+
### Rootfs creation
This section shows how to build a basic rootfs using the default distribution.
