Update readme

README.md

 
 These chips have been tested with SerialFlash:
 
+    Winbond W25Q80BV    (http://www.adafruit.com/product/1564)
     Winbond W25Q128FV
     Winbond W25Q256FV
     Micron N25Q512A
 
 Files created for erasing automatically increase in size to the nearest number of erasable blocks, resulting in a file that may be 4K to 128K larger than requested.
 
+### Delete A File
+
+    SerialFlash.remove(filename);
+
+The actual space used by the file is not reclaimed.  However, a new file with this name may be created after the original is deleted.
+
+### Check If A File Exists (without opening)
+
+    SerialFlash.exists(filename);
+
 ### Directory Listing
 
     SerialFlash.opendir();