Update README.md

Function Generator/README.md

@@ -5,11 +5,16 @@
 * Features:
   * Direct Digital Synthesis (DDS) Function Generator
   * Dual 8 bit R-2-R Digital to Analog converter
-  * Target device = Pimoroni PGA2040 (header can be modified to use Pico)
+  * Variable output level (requires MCP41020 dual digital potentiometer with SPI interface)
+  * Target device = Pico (also works on Pimoroni PGA2040)
   * External, 3 digit, SPI display
   * Independant control of either channel through Putty terminal session
-  * Putty terminal session over USB port  
-
+  * Putty terminal session over USB port
+ 
+* Limitations:
+  * USB serial requires Windows (10 or later)
+  * Square wave rise time is slow ( distortion visable at ~10KHz )
+  * Phase sync mechanism not effective at high frequencies
 
 **Two independanty controled, phase locked, DMA channels...**