Maybe add to readme: how to bind to key

Since I had to fiddle a bit (args args args args....) maybe we could add a line to the readme?

``` python
  // Number Manipulation
  { "keys": ["super+alt+up"], "command": "modify_numbers", "args":{"args":{"modifier_name" : "increment"}}},
  { "keys": ["super+alt+down"], "command": "modify_numbers", "args":{"args":{"modifier_name" : "decrement"}}},
```
