maven build doc: include

documentation/docs/maven-build.md

 # Maven Build Template
-The template `templates/mvn_build` builds your maven project and stores the resulting `jar`-File as an Gitlab Artifact for download or later use. Include it with:
 
     include:
     - project: 'devops/gitlabcitemplates/'
     ref: master
     file: 'templates/mvn_build.yml'
 
+    stages:
+    - build
+
+    maven-build:
+      extends: .build
+      stage: build
+
 ## Description
 The template builds your maven project and stores the resulting `jar` file as job artifact for download or for later use.