Added 1-2 sentences about optional dependencies.
Added 1-2 sentences about optional dependencies.

wangsheng1001/incubator-singa,wangsheng1001/incubator-singa,nusdbsystem/incubator-singa,jinyangturbo/incubator-singa,wangsheng1001/incubator-singa,cac2003/incubator-singa,venkatsatish/incubator-singa,nusdbsystem/incubator-singa,nudles/incubator-singa,ijingo/incubator-singa,apache/incubator-singa,nudles/incubator-singa,nudles/incubator-singa,jinyangturbo/incubator-singa,venkatsatish/incubator-singa,apache/incubator-singa,ijingo/incubator-singa,nudles/incubator-singa,kaiping/incubator-singa,nusdbsystem/incubator-singa,nusdbsystem/incubator-singa,apache/incubator-singa,wangsheng1001/incubator-singa,nudles/incubator-singa,cac2003/incubator-singa,kaiping/incubator-singa,wangsheng1001/incubator-singa,ijingo/incubator-singa,venkatsatish/incubator-singa,jinyangturbo/incubator-singa,ijingo/incubator-singa,kaiping/incubator-singa,ijingo/incubator-singa,kaiping/incubator-singa,venkatsatish/incubator-singa,cac2003/incubator-singa,apache/incubator-singa,jinyangturbo/incubator-singa,cac2003/incubator-singa,apache/incubator-singa,kaiping/incubator-singa,nusdbsystem/incubator-singa