chore(docs): rework instructions for OPA integration

Author: alukach

README.md

@@ -176,7 +176,7 @@ The system supports generating CQL2 filters based on request context to provide
 > 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).
+> Integration with external authorization systems (e.g. [Open Policy Agent](https://www.openpolicyagent.org/)) can be achieved by specifying an `ITEMS_FILTER` that points to a class/function that, once initialized, returns a [`cql2.Expr` object](https://developmentseed.org/cql2-rs/latest/python/#cql2.Expr) when called with the request context.
 
 #### Example GET Request Flow