Improved README.
Improved README.

liujuncheng/dockerfile-android,rednut/docker-ubuntu,pdeslaur/ubuntu,jbng/nodejs-es,mj19910923/dockerfile.ubuntu.base,dockerfile/ubuntu,retroconduct/ubuntu,the1sky/ubuntu,ruige123456/5-25,Christopher-Chopin/ubuntu,ossianpe/docker-build-centos,tianzhidao28/ubuntu