DAS plugins: add missed changelog note

Change-Id: Idcc35b2bf6e35b82ea12767c2e0f7a15c55a982f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/447402
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: srawlins

pkg/analysis_server_plugin/CHANGELOG.md

@@ -1,6 +1,8 @@
 ## 0.2.3-dev
 
 - Require version `8.2.0` of the `analyzer` package.
+- Add support for automatic re-analysis of files changed on-disk (as opposed to
+  file contents changed in the IDE, which is already supported).
 
 ## 0.2.2