Update 'README.md'

README.md

@@ -1,2 +1,35 @@
 # xbackcalc
+## Preparation
 
+### 1. Create group
+
+`sudo groupadd brightness`
+
+### 2. Add user to group
+
+`sudo usermod --append --groups video $USER`
+
+### 3. Add new udev rules
+
+Create file `/etc/udev/rules.d/90-backlight.rules` and add the following two lines:
+   
+```
+ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chgrp brightness /sys/class/backlight/%k/brightness"
+ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"
+```
+
+### 4. Apply new udev rules
+
+`sudo udevadm control --reload && sudo udevadm trigger --action=add --subsystem-match=backlight`
+
+## Examples
+
+```
+xbackcalc 10
+xbackcalc 1180
+xbackcalc current + 50
+xbackcalc current - 50
+xbackcalc current / 2
+xbackcalc current \* 1.1
+xbackcalc current \* 0.9 + 10
+```