Add nRF51 support to the readme

Author: jonas-schievink

README.md

@@ -1,6 +1,6 @@
 # `nrf-hal`
 
-> [HAL] for the nRF52 and nRF91 family of microcontrollers
+> [HAL] for the nRF51, nRF52 and nRF91 families of microcontrollers
 
 [HAL]: https://crates.io/crates/embedded-hal
 
@@ -11,6 +11,7 @@
 Support for each Nordic chip in this repository is in its own crate, with its
 own version number. Currently this repository supports:
 
+* [`nrf51-hal`](./nrf51-hal) [![docs.rs](https://docs.rs/nrf51-hal/badge.svg)](https://docs.rs/nrf51-hal) [![crates.io](https://img.shields.io/crates/d/nrf51-hal.svg)](https://crates.io/crates/nrf51-hal)
 * [`nrf52810-hal`](./nrf52810-hal) [![docs.rs](https://docs.rs/nrf52810-hal/badge.svg)](https://docs.rs/nrf52810-hal) [![crates.io](https://img.shields.io/crates/d/nrf52810-hal.svg)](https://crates.io/crates/nrf52810-hal)
 * [`nrf52832-hal`](./nrf52832-hal) [![docs.rs](https://docs.rs/nrf52832-hal/badge.svg)](https://docs.rs/nrf52832-hal) [![crates.io](https://img.shields.io/crates/d/nrf52832-hal.svg)](https://crates.io/crates/nrf52832-hal)
 * [`nrf52840-hal`](./nrf52840-hal) [![docs.rs](https://docs.rs/nrf52840-hal/badge.svg)](https://docs.rs/nrf52840-hal) [![crates.io](https://img.shields.io/crates/d/nrf52840-hal.svg)](https://crates.io/crates/nrf52840-hal)