Merge pull request #19 from ARMmbed/bulislaw_add_readme

Add README.md with firmware informations

Author: geky

README.md

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+# esp8266 mbed-os driver
+The driver for the ESP8266 WiFi module
+
+## Firmware version
+esp8266 modules come in different shapes and forms, but most important difference is which firmware version it is programmed with. To make sure that your module has mbed-os compatible firmware follow update guide: https://developer.mbed.org/teams/ESP8266/wiki/Firmware-Update