add MSRV 1.52.0 badge

- suggested by @mattico

Author: andrewgazelka

README.md

@@ -7,6 +7,7 @@ stm32h7xx-hal
 [![Bors enabled](https://bors.tech/images/badge_small.svg)](https://app.bors.tech/repositories/12691)
 [![CI](https://github.com/stm32-rs/stm32h7xx-hal/workflows/Continuous%20integration/badge.svg)](https://github.com/stm32-rs/stm32h7xx-hal/actions)
 [![Crates.io](https://img.shields.io/crates/v/stm32h7xx-hal.svg)](https://crates.io/crates/stm32h7xx-hal)
+![Minimum rustc version](https://img.shields.io/badge/rustc-1.52.0+-yellow.svg)
 
 [_stm32h7xx-hal_](https://github.com/stm32-rs/stm32h7xx-hal) contains
 a hardware abstraction layer on top of the peripheral access API for