Add release note for the addition of the remark diagnostic severity

Author: DougGregor

Release Notes/510.md

@@ -9,6 +9,10 @@
   - Description: Returns the index of the n-th element in a `SyntaxCollection`. This computation is in O(n) and `SyntaxCollection` is not subscriptable by an integer.
   - Pull Request: https://github.com/apple/swift-syntax/pull/2014
 
+- `DiagnosticSeverity` and `PluginMessage.Diagnostic.Severity` now have new case named `remark`
+  - Description: Remarks are used by the Swift compiler and other tools to describe some aspect of translation that doesn't reflect correctness, but may be useful for the user. Remarks have been added to the diagnostic severity enums to align with the Swift compiler.
+  - Pull Request: https://github.com/apple/swift-syntax/pull/2143
+
 ## API Behavior Changes
 
 ## Deprecations