Add reference to OptaPlanner starter

See gh-19508

Author: ge0ffrey

spring-boot-project/spring-boot-starters/README.adoc

@@ -154,6 +154,9 @@ do as they were designed before this was clarified.
 | https://developer.okta.com/[Okta]
 | https://github.com/okta/okta-spring-boot
 
+| https://www.optaplanner.org/[OptaPlanner]
+| https://github.com/kiegroup/optaplanner/tree/master/optaplanner-integration/optaplanner-spring-boot-starter
+
 | https://orika-mapper.github.io/orika-docs/[Orika]
 | https://github.com/akihyro/orika-spring-boot-starter