Added info about boost to the readme
Added info about boost to the readme


Former-commit-id: d0ffc3abdd81405badb5e9f5e702760fd790b4da
danielhers/dynet,shuheik/dynet,shuheik/dynet,xunzhang/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,clab/cnn,clab/dynet,shuheik/dynet,chunyang-wen/dynet,chunyang-wen/dynet,clab/cnn,danielhers/dynet,chunyang-wen/dynet,clab/dynet,clab/dynet,xunzhang/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,clab/dynet,shuheik/dynet,xunzhang/dynet,shuheik/dynet,chunyang-wen/dynet,clab/cnn,clab/dynet