packagers: btrfs build tag docs

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)

hack/PACKAGERS.md

@@ -160,15 +160,23 @@ export DOCKER_BUILDTAGS='apparmor'
 There are build tags for disabling graphdrivers as well. By default, support
 for all graphdrivers are built in.
 
-To disable devicemapper
+To disable btrfs:
+```bash
+export DOCKER_BUILDTAGS='exclude_graphdriver_btrfs'
+```
+
+To disable devicemapper:
 ```bash
 export DOCKER_BUILDTAGS='exclude_graphdriver_devicemapper'
 ```
-To disable aufs
+
+To disable aufs:
 ```bash
 export DOCKER_BUILDTAGS='exclude_graphdriver_aufs'
 ```
 
+NOTE: if you need to set more than one build tag, space seperate them.
+
 ### Static Daemon
 
 If it is feasible within the constraints of your distribution, you should