Mention that the test JAR is checked in

spring-auto-restdocs-docs/contributing.adoc

@@ -4,11 +4,26 @@
 [[contributing-building]]
 === Building from source
 
+[[contributing-installing-testjar]]
+=== Install Spring REST Docs test JAR
+
+The Spring REST Docs test JAR is not published via Maven, but this project relies on it.
+If you want to build this project yourself, you need to install the test JAR.
+The test JAR is located in the lib folder and can be installed as follow:
+
+.Bash
+[source,bash]
+----
+mvn install:install-file -Dfile=lib/spring-restdocs-core-${spring-restdocs.version}-test.jar \
+    -DgroupId=org.springframework.restdocs -DartifactId=spring-restdocs-core \
+    -Dversion=${spring-restdocs.version} -Dpackaging=test-jar -Dclassifier=test
+----
+
 [[contributing-building-testjar]]
-==== Get spring-restdocs-core test JAR
+==== Build Spring REST Docs test JAR
 
-The test JAR is not published, but this project relies on it.
-If you want to build this project yourself, you first have to build and copy the test JAR on your system.
+Building the Spring REST Docs test JAR is not required to build the project,
+but if you ever want to upgrade the version of Spring REST Docs in this project this step has to be done.
 
 We use version ${spring-restdocs.version} of Spring REST Docs in this example.
 
@@ -38,11 +53,9 @@ git checkout tags/v${spring-restdocs.version}
 ./gradlew build
 ----
 
-Afterwards you copy
+Afterwards the test JAR is located at
 `spring-restdocs/spring-restdocs-core/build/libs/spring-restdocs-core-${spring-restdocs.version}-test.jar`
-to
-`~/.m2/repository/org/springframework/restdocs/spring-restdocs-core/${spring-restdocs.version}`
-so that the test JAR is available to Maven.
+and has to be installed with the Maven command shown in the section above.
 
 [[contributing-building-build]]
 ==== Build