Add links to development boards

Author: dnorthcote

README.md

@@ -1,7 +1,7 @@
 <img src="strathclyde_banner.png" width="100%">
 
 # RFSoC Introductory Notebooks
-A collection of RFSoC introductory notebooks. This repository is compatible with [PYNQ image v2.6](https://github.com/Xilinx/PYNQ/releases) for the ZCU111 and RFSoC2x2 development board.
+A collection of RFSoC introductory notebooks. This repository is only compatible with [PYNQ images v2.6](https://github.com/Xilinx/PYNQ/releases) for the [ZCU111](https://www.xilinx.com/products/boards-and-kits/zcu111.html) and [RFSoC2x2](http://rfsoc-pynq.io/).
 
 <img src="./nb_rf_data_converters.png" width="25%" align="left" />
 <img src="./nb_rf_spectrum.png" width="25%" align="left" />