Update README.md

README.md

@@ -27,5 +27,47 @@ To install, run:
 pip install simple-pid
 ```
 
+## Usage
+The `PID` class implements `__call__()`, which means that to compute a new output value, you simply call the object like this:
+```python
+output = pid(current_value)
+```
+
+The PID works best when it is updated at regular intervals. To achieve this, set `sample_time` to the amount of time there should be between each update and then call the PID every time in the program loop. A new output will only be calculated when `sample_time` seconds has passed:
+```python
+pid.sample_time = 0.01  # update every 0.01 seconds
+
+while True:
+    output = pid(current_value)
+```
+
+To set the setpoint, ie. the value that the PID is trying to achieve, simply set it like this:
+```python
+pid.setpoint = 10
+```
+
+The tunings can be changed any time when the PID is running. They can either be set individually or all at once:
+```python
+pid.Ki = 1.0
+pid.tunings = (1.0, 0.2, 0.4)
+```
+
+To disable the PID so that no new values are computed, set auto mode to False:
+```python
+pid.auto_mode = False  # no new values will be computed when pid is called
+pid.auto_mode = True   # pid is enabled again
+```
+
+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
+pid.output_limits = (0, None)  # output will always be above 0, but with no upper bound
+```
+
+To eliminate overshoot in certain types of systems, you can calculate the [proportional term directly on the measurement](http://brettbeauregard.com/blog/2017/06/introducing-proportional-on-measurement/) instead of the error. This can be enabled like this:
+```python
+pid.proportional_on_measurement = True
+```
+
 ## License
 Licensed under the [MIT License](https://github.com/m-lundberg/simple-pid/blob/master/LICENSE.md).