chore(docs): describe Features Filter requirement for item filter

Author: alukach

README.md

@@ -173,7 +173,7 @@ The middleware stack is processed in reverse order (bottom to top):
 The system supports generating CQL2 filters based on request context to provide row-level content filtering. These CQL2 filters are then set on outgoing requests prior to the upstream API.
 
 > [!IMPORTANT]
-> The upstream STAC API must support the [STAC API Filter Extension](https://github.com/stac-api-extensions/filter/blob/main/README.md).
+> The upstream STAC API must support the [STAC API Filter Extension](https://github.com/stac-api-extensions/filter/blob/main/README.md), including the [Features Filter](http://www.opengis.net/spec/ogcapi-features-3/1.0/conf/features-filter) conformance class on to the Features resource (`/collections/{cid}/items`) [#37](https://github.com/developmentseed/stac-auth-proxy/issues/37).
 
 > [!TIP]
 > Integration with external authorization systems (e.g. [Open Policy Agent](https://www.openpolicyagent.org/)) can be achieved by replacing the default `BuildCql2FilterMiddleware` with a custom async middleware that is capable of generating [`cql2.Expr` objects](https://developmentseed.org/cql2-rs/latest/python/#cql2.Expr).