doc(port_riscv): update changelog to mention the addition of ThreadingOptions::PRIVILEGE_LEVEL

Author: yvt

src/r3_port_riscv/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+### Added
+
+- The new option `ThreadingOptions::PRIVILEGE_LEVEL` allows for running the kernel in other privilege levels than M-mode.
+
 ### Fixed
 
 - The default stack alignment (`PortThreading::STACK_ALIGN`) now conforms to the standard ABI requirement (128-bit alignment).