Update README.md

Author: deanm1278

README.md

@@ -3,6 +3,8 @@
 ## Known Issues:
 * gcc optimizations must be off for sketches to work consistently. We are working to track this down.
 * AREF must be tied to 3.3V for dac to work. This is a bug in the SAMD51 silicon.
+* USB host mode doesn't work yet
+* I2S doesn't work yet
 
 This repository contains the source code and configuration files of the Arduino Core
 for Atmel's SAMD21 and SAMD51 processor (used on the Arduino/Genuino Zero, MKR1000 and MKRZero boards).