Updated pom.xml so the source jar gets installed or deployed to the maven repository

This change was provided so whenever the file: docopt-0.6.0-SNAPSHOT.jar gets installed or deployed, the file: docopt-0.6.0-SNAPSHOT-sources.jar goes along with it.

Doing this made it so I could create a new project which uses docopt.java, and maven would automatically pull down the sources and attach them to the libraries when I generated my eclipse project as follows:
> mvn eclipse:eclipse -DdownloadSources=true

I hope you find this change as useful as I did.