chore: update README

Author: Junius00

README.md

@@ -28,6 +28,9 @@ k6_N10_mem32K_40nm/ch_intrinsics                OK
 k6_N10_mem32K_40nm/ch_intrinsics                OK
 ```
 
+## Special Notes
+- F4PGA is currently incompatible with the latest version of Yosys (see issue [here](https://github.com/chipsalliance/yosys-f4pga-plugins/issues/552)); manually downgraded to v0.38. Upgrade when the issue is fixed.
+
 # Verilog to Routing (VTR)
 [![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-Ready--to--Code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/verilog-to-routing/vtr-verilog-to-routing.git)
 [![Build Status](https://github.com/verilog-to-routing/vtr-verilog-to-routing/workflows/Test/badge.svg)](https://github.com/verilog-to-routing/vtr-verilog-to-routing/actions?query=workflow%3ATest) [![Documentation Status](https://readthedocs.org/projects/vtr/badge/?version=latest)](http://docs.verilogtorouting.org/en/latest/)