Update README.txt to .md and add Astrom reference

Author: David Forrest

README.txt renamed to README.md

@@ -9,3 +9,5 @@
    http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/
 
  - For function documentation see:  http://playground.arduino.cc/Code/PIDLibrary
+
+This fork uses back calculation per [Astrom 1989](http://cse.lab.imtlucca.it/~bemporad/teaching/controllodigitale/pdf/Astrom-ACC89.pdf) to manage integral windup.