Add documentation to the cascading PID logic.

README.md

@@ -79,6 +79,28 @@ climate:
     pwm: 0
 ```
 
+```
+climate:
+  - platform: smart_thermostat
+    name: Smart Thermostat Cascading PID Example
+    unique_id: smart_thermostat_outer_pid_example
+    heater: climate.smart_thermostat_single_on_off_heat_example
+    target_sensor: sensor.ambient_temperature2
+    min_temp: 7
+    max_temp: 28
+    ac_mode: False
+    target_temp: 19
+    keep_alive:
+      seconds: 60
+    away_temp: 14
+    kp: 5
+    ki: 0.01
+    kd: 500
+    output_min: 7
+    output_max: 28
+    pwm: 0
+```
+
 ## Usage:
 The target sensor measures the ambient temperature while the heater switch controls an ON/OFF 
 heating system.\
@@ -123,6 +145,15 @@ PID output value is the weighted sum of the control terms:\
 `output = P + I + D`\
 Output is then limited to 0% to 100% range to control the PWM.
 
+#### Cascading PID
+Optionally, 2 (or more) PIDs can be used in a cascading matter, e.g. for underfloor heating define an outer
+PID between the room temperature and the floor temperature, and an inner PID between the floor
+temperature and the PWM. See details [here](
+https://en.wikipedia.org/wiki/Proportional%E2%80%93integral%E2%80%93derivative_controller#Cascade_control).
+
+To enable, create the inner thermostate as detailed above, and then create another thermostate that will
+control the inner one.
+
 #### Outdoor temperature compensation
 Optionally, when an outdoor temperature sensor entity is provided and ke is set, the thermostat can 
 automatically compensate building losses based on the difference between target temperature and