Add WARNING about downloaded OpenNLP models during build

We download some OpenNLP file during the build process. We should warn the user that we do this and that these files have a more restrictive license than AL20.

Something like (from Bertrand)

WARNING - this build downloads some OpenNLP files that are not
licensed under the Apache License, and have more restrictive usage
terms than the Stanbol code.

See http://markmail.org/thread/5n2gletdzo5k73g6