Port "add -R explanation in README (and mention that generally it's not needed)"
Port "add -R explanation in README (and mention that generally it's not needed)"

Ported from https://github.com/cjlin1/liblinear/commit/c3af75aa386b91030b36b2b8f9c8a61ba2f23d2a

bwaldvogel/liblinear-java,bwaldvogel/liblinear-java