Add CHANGELOG.md

Author: KushalMeghani1644

CHANGELOG.md

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+# Changelog
+
+## [Unreleased]
+
+### Added
+- Added support for the RISC-V `dcsr` (Debug Control and Status Register) CSR.
+  - Created `src/register/dcsr.rs`
+  - Exposed the module in `register.rs`