Resolve "OpenAPI in dokument einpflegen"

Closes #11 (closed)

chapters/documentation.md

@@ -57,8 +57,15 @@ Call diagrams can be useful to follow code and service calls and should be exist
 - Links to callers must not be listed in the documentation, because this info will be deprecated soon. It is strongly
 recommended to use call stacks of tools like Eclipse (Java) and/or Call Graph Module (SADE).
 
-- Document REST-APIs using [openAPI](https://github.com/OAI/OpenAPI-Specification) if possible. OpenAPI docs should be
-located at `/doc/api` on servers.
+- Document REST-APIs using [OpenAPI](https://github.com/OAI/OpenAPI-Specification) if possible.
+OpenAPI docs should be located at `/doc/api` on servers.
+We currently use OpenAPI for RESTXQ and JAX-RS service endpoints.
+
+##### OpenAPI for RESTXQ
+
+For documenting RESTXQ APIs in our eXist-db systems we use the [OpenAPI4RESTXQ](https://gitlab.gwdg.de/subugoe/openapi4restxq)
+application.
+It simply has to be installed in eXist-db and configured according to the [instructions](https://gitlab.gwdg.de/subugoe/openapi4restxq#use).
 
 #### CESSDA's Software Maturity Levels in RDD (CA1.3)