esp8266/README: Add notice about 512K version.

esp8266/README.md

@@ -79,6 +79,16 @@ $ make PORT=/dev/ttyUSB0 FLASH_MODE=qio FLASH_SIZE=32m deploy
 
 The image produced is `build/firmware-combined.bin`, to be flashed at 0x00000.
 
+__512KB FlashROM version__
+
+The normal build described above requires modules with at least 1MB of FlashROM
+onboard. There's a special configuration for 512KB modules, which can be
+built with `make 512k`. This configuration is highly limited, lacks filesystem
+support, WebREPL, and has many other features disabled. It's mostly suitable
+for advanced users who are interested to fine-tune options to achieve a required
+setup. If you are an end user, please consider using a module with at least 1MB
+of FlashROM.
+
 First start
 -----------