- added ziegler nichols method of tuning

docs/pid_tuning.md

@@ -51,3 +51,4 @@ Now, run the test schedule again and see how well it works.  Expect some oversho
 * only change one value at a time, then test it.
 * If there is too much overshoot, decrease pid_kp.
 * If the temp is always below the set point, increase pid_ki.
+* if the above does not work, try the Ziegler / Nichols method https://blog.opticontrols.com/archives/477