Update README.md

README.md

@@ -58,6 +58,14 @@ pid.Ki = 1.0
 pid.tunings = (1.0, 0.2, 0.4)
 ```
 
+To use the PID in [reverse mode](http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-direction/), meaning that an increase in the input leads to a decrease in the output (like when cooling for example), you can set the tuning to negative values:
+
+```python
+pid.tunings = (-1.0, -0.1 0)
+```
+
+Note that all the tunings should have the same sign.
+
 In order to get output values in a certain range, and also to avoid [integral windup](https://en.wikipedia.org/wiki/Integral_windup) (since the integral term will never be allowed to grow outside of these limits), the output can be limited to a range:
 ```python
 pid.output_limits = (0, 10)    # output value will be between 0 and 10