Add ETag-only cache mode

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 # Version 2
 
+## 2.1.0 - TBD
+
+### Added
+
+- Added `EtagCachePlugin` to cache only ETag-backed responses and always revalidate them before serving cached bodies.
+
 ## 2.0.2 - 2025-12-01
 
 - Support Symfony 8.

README.md

@@ -23,6 +23,14 @@ composer require php-http/cache-plugin
 
 Please see the [official documentation](http://docs.php-http.org/en/latest/plugins/cache.html).
 
+To only cache ETag-backed responses and always revalidate cached bodies with `If-None-Match`, use `EtagCachePlugin`:
+
+``` php
+$plugin = EtagCachePlugin::clientCache($pool, $streamFactory);
+```
+
+Responses without an `ETag` header are not cached. Cached responses are only returned after the origin replies with `304 Not Modified`.
+
 
 ## Testing