added coverity badge

README.md

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 ![tinyUSB_240x100](https://user-images.githubusercontent.com/249515/62646655-f9393200-b978-11e9-9c53-484862f15503.png)
 
-[![Build Status](https://travis-ci.org/hathach/tinyusb.svg?branch=master)](https://travis-ci.org/hathach/tinyusb) [![License](https://img.shields.io/badge/license-MIT-brightgreen.svg)](https://opensource.org/licenses/MIT)
+[![Build Status](https://travis-ci.org/hathach/tinyusb.svg?branch=master)](https://travis-ci.org/hathach/tinyusb) [![License](https://img.shields.io/badge/license-MIT-brightgreen.svg)](https://opensource.org/licenses/MIT)[![Coverity](https://img.shields.io/coverity/scan/458.svg)](https://scan.coverity.com/projects/tinyusb)
 
 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.