add ubuntu install instructions

README.md

@@ -18,6 +18,15 @@ Many little improvements have been made to i3lock over time:
 - i3lock uses PAM and therefore is compatible with LDAP etc.
   On OpenBSD i3lock uses the bsd_auth(3) framework.
 
+Running i3lock
+-------------
+Ubuntu:
+```
+sudo apt-get install i3lock
+```
+
+For other operating systems, see Requirements and Building below.
+
 Requirements
 ------------
 - pkg-config