readme: added docs status badge

Signed-off-by: perigoso <perigoso@riseup.net>

README.md

@@ -1,6 +1,6 @@
 ![TinyUSB](docs/assets/logo.svg)
 
-[![Build Status](https://github.com/hathach/tinyusb/workflows/Build/badge.svg)](https://github.com/hathach/tinyusb/actions) [![License](https://img.shields.io/badge/license-MIT-brightgreen.svg)](https://opensource.org/licenses/MIT)
+[![Build Status](https://github.com/hathach/tinyusb/workflows/Build/badge.svg)](https://github.com/hathach/tinyusb/actions) [![Documentation Status](https://readthedocs.org/projects/tinyusb/badge/?version=latest)](https://openinput.readthedocs.io/en/latest/?badge=latest) [![License](https://img.shields.io/badge/license-MIT-brightgreen.svg)](https://opensource.org/licenses/MIT)
 
 TinyUSB is an open-source cross-platform USB Host/Device stack for embedded system, designed to be memory-safe with no dynamic allocation and thread-safe with all interrupt events are deferred then handled in the non-ISR task function.