add note about boot_v1.5 bug

FLASHING.md

@@ -100,6 +100,10 @@ flash modules. Note the different address for esp_init_data_default.bin and blan
 
 For __8Mbit / 1MByte__ modules the addresses are 0xFC000 and 0xFE000.
 
+__Warning__: there is a bug in boot_v1.5.bin which causes it to only boot into user1 once.
+If that fails it gets stuck trying to boot into user2. If this happens (can be seen in the
+boot output on uart2 at 76600 baud) reflash just blank.bin at 0x7E000 (4Mbit module). (Sigh)
+
 ## Updating the firmware over-the-air
 
 This firmware supports over-the-air (OTA) flashing for modules with 1MByte or more flash,