bump CHANGELOG to 3.6.0
bump CHANGELOG to 3.6.0

gojomo/gensim,RaRe-Technologies/gensim,piskvorky/gensim,RaRe-Technologies/gensim,RaRe-Technologies/gensim,gojomo/gensim,RaRe-Technologies/gensim,piskvorky/gensim,gojomo/gensim,gojomo/gensim,piskvorky/gensim