add usage info

README.md

@@ -1,3 +1,41 @@
 # awesome-powermenu
 
 Simple popup power menu for awesomewm
+
+## Usage
+
+`require` the library:
+
+```
+local powermenu = require "awesome-powermenu"
+```
+
+Initialise the menu:
+
+```
+powermenu.new()
+```
+
+Bind it to something:
+
+```
+global_key({modkey}, "p", function() powermenuPopup:toggle() end,
+           "show power menu", "launcher")
+```
+
+## Configuration
+
+```
+powermenu.buttonsize = 128,
+powermenu.spacing = 5,
+powermenu.iconPath = gears.filesystem.get_configuration_dir() .. "/powermenu/",
+powermenu.lock = function()
+ -- your lock function here
+end,
+powermenu.suspend = function()
+ -- your suspend function here
+end
+powermenu.poweroff = function()
+ -- your power off function here
+end
+```