Add a module-info.java to enable the use of jlink

[tcfurrer]

Although #134 took the first step in the direction of providing basic support for JPMS modularity, there is still a problem for modular apps with a dependency on lz4-java. The jlink tool does not allow dependencies upon automatic modules. Although it is possible for these apps to workaround this using jdeps --generate-module-info, it would be nice to eliminate their need for that awkward workaround by simply creating a module-info.java to be included in the released .jar file.