docs(README): Add readdocs link to README
+# ESP8266 RTOS Software Development Kit
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+[![Documentation Status](https://readthedocs.com/projects/espressif-esp8266-rtos-sdk/badge/?version=latest)](https://docs.espressif.com/projects/esp8266-rtos-sdk/en/latest/?badge=latest)
+
+
# ** IMPORTANT NOTICE **
## About this repository
