write readme

README.md

@@ -1 +1,41 @@
 # write-intel-brightness
+
+This is a script to control screen brightness on Lenovo ThinkPad X220 and similar after you changed PWM frequency by writing a custom value to 0xC8254 register. I use it with acpid.
+
+The script reads current PWM frequency from the register, calculates correct new brightness level for `/sys/class/backlight/intel_backlight/brighness` and writes it.
+
+### Usage
+
+To increase brightness:
+```
+write-intel-brightness +
+```
+
+To decrease:
+```
+write-intel-brightness -
+```
+
+### acpid example
+
+From `/etc/acpi/default.sh` on my Gentoo box:
+
+```
+case "$group" in
+    video)
+        case "$action" in
+            brightnessup)
+                /usr/local/bin/write-intel-brightness +
+                ;;
+
+            brightnessdown)
+                /usr/local/bin/write-intel-brightness -
+                ;;
+        esac
+        ;;
+esac
+```
+
+### License
+
+MIT