Add warning regarding environment variables

README.md

@@ -3,6 +3,10 @@
 [![Build status](https://ci.appveyor.com/api/projects/status/r943gjn189rlxts9/branch/main?svg=true)](https://ci.appveyor.com/project/ashleyfrieze/system-stubs/branch/main)
  [![codecov](https://codecov.io/gh/webcompere/system-stubs/branch/main/graph/badge.svg?token=J0N9VCXFQ1)](https://codecov.io/gh/webcompere/system-stubs)
 
+> **⚠ WARNING: JDK Compatibility.**  
+> From JDK16 onwards, there's deep restrictons on the ability to use reflection to modify the Unmodifiable `Map` where `System` stores the
+> environment variables. We're looking for a fix to this. Until then, this library is not recommended for environment variable settings.
+
 ## Overview
 System Stubs is used to test code which depends on methods in `java.lang.System`.