POM: add a note about sonatype.public repo
POM: add a note about sonatype.public repo

See: https://gitter.im/scenerygraphics/SciView?at=5ae366126d7e07082b3f69b8

kephale/ThreeDViewer,scenerygraphics/SciView,kephale/ThreeDViewer,scenerygraphics/SciView