Add comments for Ultimainboard 2.0 configuration.

Marlin/Configuration.h

 // 68 = Azteeg X3 Pro
 // 7  = Ultimaker
 // 71 = Ultimaker (Older electronics. Pre 1.5.4. This is rare)
+// 72 = Ultimainboard 2.x (Uses TEMP_SENSOR 20)
 // 77 = 3Drag Controller
 // 8  = Teensylu
 // 80 = Rumba
 // 8 is 100k 0603 SMD Vishay NTCS0603E3104FXT (4.7k pullup)
 // 9 is 100k GE Sensing AL03006-58.2K-97-G1 (4.7k pullup)
 // 10 is 100k RS thermistor 198-961 (4.7k pullup)
+// 20 is the PT100 circuit found in the Ultimainboard V2.x
 // 60 is 100k Maker's Tool Works Kapton Bed Thermister
 //
 //    1k ohm pullup tables - This is not normal, you would have to have changed out your 4.7k for 1k