add logging options documentation

Author: hauner

docs/modules/ROOT/pages/processor/configuration.adoc

@@ -29,6 +29,9 @@ options:
   base-path:
     server-url: 0
     properties-name: api.properties
+logging:
+  mapping: true
+  mapping-target: stdout
 compatibility:
   bean-validation-valid-on-reactive: false
   identifier-word-break-from-digit-to-letter: false
@@ -378,6 +381,40 @@ options:
     layout: standard
 ----
 
+== logging:
+
+This section contains keys to log the mapping lookup. It may be useful to locate mappings that should be used, but are not.
+
+=== mapping:
+
+**optional** (boolean, `true` or `false`, default is `false`)
+
+enables logging of the mapping lookup.
+
+=== mapping-target:
+
+**optional** (string, `logger` or `stdout`, default is `logger`)
+
+with this option it is possible to control the logging target.
+
+If set to `logger` the mapping lookup gets logged at `info` level to link:https://www.slf4j.org/[slf4j]. If set to `stdout` the mapping lookup gets written directly to `stdout` without slf4j.
+
+==== maven
+
+apart from enabling logging of the mapping lookups in the `mapping.yaml` it is necessary to enable the mapping logger with `io.openapiprocessor.core.converter.mapping` to see any output.
+
+For example by running `maven` with:
+
+----
+./mvnw -Dorg.slf4j.simpleLogger.log.io.openapiprocessor.core.converter.mapping=info compile -f pom.xml
+----
+
+==== gradle
+
+apart from enabling logging of the mapping lookups in the `mapping.yaml` it is necessary to set `mapping-target` to `stdout`.
+
+gradle can only globally enable log levels which is very noisy. The best option to log the mapping lookups is simply write them to stdout.
+
 == compatibility:
 
 This section contains keys to disable breaking changes.