Add link to RP2040 board support crates full list.

Author: tim-seoss

README.md

@@ -667,6 +667,8 @@ Crates tailored for specific boards.
 
 - [`rp-pico`](https://github.com/rp-rs/rp-hal-boards) - Board Support Crate for the RP2040-based Raspberry Pi Pico.
 
+The same repository includes support for numerous other development boards which use Raspberry Pi microcontrollers.  A full list can be found [in the repository documentation](https://github.com/rp-rs/rp-hal-boards/blob/main/README.md#packages).
+
 ### Sparkfun
 
 - [`samd21_mini`](https://crates.io/crates/samd21_mini) - Board support for the [SAMD21 Mini Breakout](https://www.sparkfun.com/products/13664) in the [atsamd-rs] repo. It is an [atsamd-rs tier 2 support] board. [![crates.io](https://img.shields.io/crates/v/samd21_mini.svg)](https://crates.io/crates/samd21_mini)