add "unsafe forbidden" shield to README

Author: rursprung

README.md

@@ -2,6 +2,7 @@
 [![CI](https://github.com/rursprung/adafruit-bluefruit-protocol-rs/actions/workflows/CI.yml/badge.svg)](https://github.com/rursprung/adafruit-bluefruit-protocol-rs/actions/workflows/CI.yml)
 [![Crates.io](https://img.shields.io/crates/v/adafruit-bluefruit-protocol)](https://crates.io/crates/adafruit-bluefruit-protocol)
 ![Licenses](https://img.shields.io/crates/l/adafruit-bluefruit-protocol)
+[![unsafe forbidden](https://img.shields.io/badge/unsafe-forbidden-success.svg)](https://github.com/rust-secure-code/safety-dance/)
 
 This implements the [Adafruit Bluefruit LE Connect controller protocol](https://learn.adafruit.com/bluefruit-le-connect/controller)
 which is e.g. used by the [Adafruit Bluefruit LE UART Friend](https://learn.adafruit.com/introducing-the-adafruit-bluefruit-le-uart-friend).