Added more warnings about FlashIAP

Author: geky

README.md

@@ -2,9 +2,13 @@
 Block device driver using the mbed OS FlashIAP API.
 
 # Warning 
+This driver is **EXPERIMENTAL** and improper usage could kill your board's flash.
+
 This driver should only be used on platforms where the FlashIAP implementation is using external flash or in conjunction with a filesystem with wear leveling, that can operate on a page size granularity.
 
-The BlockDevice API assumes a uniform erase size so the underlying flash must also have uniform sectors.
+Additional concerns:
+- The BlockDevice API assumes a uniform erase size so the underlying flash must also have uniform sectors.
+- The FlashIAP may freeze code execution for a long period of time while writing to flash. Not even high-priority irqs will be allowed to run, which may interrupt background processes.
 
 # Tested on