Update README.md

Adds dt to README

Author: emersion

README.md

@@ -12,9 +12,10 @@
 Let's take the example of a car cruise control.  We want the car driving at 120km/h.
 
 ### Create a Controller instance
+`k_p`, `k_i` and `k_d` are the proportional, integral and derivative terms, `dt` is the interval of time between two measures.
 ```js
 var Controller = require('node-pid-controller');
-var ctr = new Controller(0.25, 0.01, 0.01); // k_p, k_i, k_d
+var ctr = new Controller(0.25, 0.01, 0.01, 1); // k_p, k_i, k_d, dt
 ```
 
 ### Set the target
@@ -46,4 +47,4 @@ mocha test
 
 ## Author
 
-Philmod <[email protected]>
+Philmod <[email protected]>