thermal: added some comments

We still stick to ACPI thermal zones because they are most commonly
exposed (with proper ACPI modules loaded). But if you can find another
source of temperature exposed trough /sys use it. Current code should
match a lot of sources, but in some cases you will want to modify it a
bit, add a dot, or limit to two numbers (except when the value is
100+, you don't want to miss the fact your CPU is melting).

thermal.lua

@@ -15,8 +15,11 @@ module("vicious.thermal")
 
 -- {{{ Thermal widget type
 local function worker(format, thermal_zone)
-    -- Get thermal zone
+    -- Get an ACPI thermal zone
     local f = io.open("/proc/acpi/thermal_zone/" .. thermal_zone .. "/temperature")
+    -- Fix your ACPI setup, or find another source of temperature
+    -- exposed trough /sys, if a thermal_zone can't be found.
+    if not f then return {"N/A"} end
     local line = f:read("*line")
     f:close()