Update readme

README.md

@@ -212,17 +212,19 @@ The image should be of the following format:
 
 Image header format includes two bytes, `F1` and `F2`, which desribe
 SPI flash parameters that ROM bootloader uses to load the rest of the firmware.
-Those two bytes encode three parameters:
 
-- Flash mode (F1 byte - can be `0`, `1`, `2`, `3`)
-- FLash size (hight 4 bits of F2 byte - can be `0`, `1`, `2`, `3`, `4`)
-- Flash frequency (low 4 bits of F2 byte - can be `0`, `1`, `2`, `f`)
-
-By default, `esputil` fetches flash params `F1` and `F2` from the
-existing bootloader by reading first 4 bytes of the bootloader from flash.
-It is possible to manually set flash params via the `-fp`
-flag, which is an integer value that represent 3 hex nimbles.
-For example `fp 0x22f` sets flash to DIO, 4MB, 80MHz:
+- Flash mode. F1 byte. Can be `0` - qio, `1` - qout, `2` - dio, `3` - dout
+- FLash size. High 4 bits of F2 byte. For ESP32: `0` - 1m, `1` - 2m, `2` -
+  4m, `3` - 8m, `4` - 16m. For ESP8266: `0` - 512k, `1` - 256k, `2` - 1m, `3`
+  - 2m, `4` - 4m, `8` - 8m, `9` - 16m
+- Flash frequency. Low 4 bits of F2 byte. Can be `0` - 40m, `1` - 26m, `2` -
+  20m, `f` - 80m
+
+By default, `esputil` fetches flash params `F1` and `F2` from the existing
+bootloader by reading first 4 bytes of the bootloader from flash.  It is
+possible to manually set flash params via the `-fp 0xABC` command line flag,
+where A is flash mode, B is flash size, C is flash frequency.  For example `fp
+0x220` sets flash to DIO, 4MB, 40MHz:
 
 ```sh
 $ esputil -fp 0x220 flash 0 firmware.bin