Add custom load balancer policies to xDS features doc (#32530)

@markdroth

doc/grpc_xds_features.md

@@ -68,3 +68,4 @@ Support for [xDS v2 APIs](https://www.envoyproxy.io/docs/envoy/latest/api/api_su
 [Security](https://www.envoyproxy.io/docs/envoy/latest/configuration/security/security):<br>Uses [certificate providers](https://github.com/grpc/proposal/blob/master/A29-xds-tls-security.md#certificate-provider-plugin-framework) instead of SDS | [A29](https://github.com/grpc/proposal/blob/master/A29-xds-tls-security.md) | v1.41.0<br>(C++ and Python) | v1.41.0 | 1.41.0 | |
 [Authorization (RBAC)](https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/filters/http/rbac/v3/rbac.proto):<br><ul><li>`LOG` action has no effect<li>CEL unsupported and rejected</ul> | [A41](https://github.com/grpc/proposal/blob/master/A41-xds-rbac.md) | v1.51.0<br>(C++ and Python) | v1.42.0 | 1.42.0 | |
 [Outlier Detection](https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/outlier):<br>Only the following detection types are supported:<ul><li>Success Rate</li><li>Failure Percentage</li></ul> | [A50](https://github.com/grpc/proposal/blob/master/A50-xds-outlier-detection.md) | v1.51.0 | v1.49.0 | v1.50.0 | v1.7.0 | |
+[Custom Load Balancer Configuration](https://github.com/envoyproxy/envoy/blob/57be3189ffa3372b34e9480d1f02b2d165e49077/api/envoy/config/cluster/v3/cluster.proto#L1208)<br> | [A52](https://github.com/grpc/proposal/blob/master/A52-xds-custom-lb-policies.md) | | v1.47.0 | | |