Add changelog entry for improved raw HTML post-processor perfs

Author: pawamoy

docs/changelog.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * DRY fix in `abbr` extension by introducing method `create_element` (#1483).
 * Clean up test directory some removing some redundant tests and port
   non-redundant cases to the newer test framework.
+* Improved performance of the raw HTML post-processor (#1510).
 
 ### Fixed