Also watch root partition

At least once I ran into problems, not realizing that full / was the
cause.

Not funny.

dotfiles/i3status.conf

@@ -15,6 +15,7 @@ order += "battery 0"
 order += "battery 1"
 order += "load"
 order += "cpu_usage"
+order += "disk /"
 order += "disk /home"
 order += "volume master"
 order += "tztime local"
@@ -49,6 +50,12 @@ tztime local {
     format = "%Y-%m-%d %H:%M"
 }
 
+disk "/" {
+    format = "/:%avail"
+    threshold_type = "gbytes_avail"
+    low_threshold = 2
+}
+
 disk "/home" {
     format = "~:%avail"
     threshold_type = "gbytes_avail"