[hotfix][docs] Adds clean to Maven command in README file

The build should be done on a fresh checkout of the flink-shaded repository. Otherwise,
we might run into errors in the jackson submodule: Error creating shaded jar: duplicate
entry: META-INF/services/org.apache.flink.shaded.jackson2.com.fasterxml.jackson.core.JsonFactory

I didn't check where this error is coming from. But building the sources based
on a clean version sounds reasonable in general.

README.md

@@ -31,7 +31,7 @@ When using these dependencies it is recommended to work directly against the sha
 
 We currently do not release jars containing the shaded sources due to the unanswered legal questions raised [here](https://github.com/apache/flink-shaded/issues/25).
 
-However, it is possible to build these jars locally by cloning the repository and calling `mvn package -Dshade-sources`.
+However, it is possible to build these jars locally by cloning the repository and calling `mvn clean package -Dshade-sources`.
 
 ## About