added a comment on query plan caching. (Azure#24091)

Author: RaviTella

sdk/cosmos/azure-spring-data-cosmos/README.md

@@ -294,6 +294,9 @@ public interface UserRepository extends CosmosRepository<User, String> {
 
 - `findByFirstName` method is custom query method, it will find items per firstName.
 
+#### Query Plan Caching
+When query plan caching is enabled, custom query methods like `findByFirstName(String firstName)` where `firstName` is the partition key will result in lower query execution time. Query plan caching can be enabled by setting the `COSMOS.QUERYPLAN_CACHING_ENABLED` System property to 'true'. Currently, query plan caching is only supported for custom query methods targeting a single partition.
+
 #### QueryAnnotation : Using annotated queries in repositories
 Azure spring data cosmos supports specifying annotated queries in the repositories using `@Query`.
 - Examples for annotated queries in synchronous CosmosRepository: