Update release notes

Author: plemarquand

Release Notes/600.md

@@ -75,6 +75,9 @@
   - Description: With the change to parse `#if canImport(MyModule, _version: 1.2.3)` as a function call instead of a dedicated syntax node, `1.2.3` natively gets parsed as a member access `3` to the `1.2` float literal. This property allows the reinterpretation of such an expression as a version tuple.
   - Pull request: https://github.com/apple/swift-syntax/pull/2025
 
+- `TokenSyntax.identifier`
+  - Description: Adds an `identifier` property to `TokenSyntax` which returns a canonicalized representation of an identifier that strips away backticks.
+
 ## API Behavior Changes
 
 ## Deprecations