1.教程
https://www.vpser.net/build/letsencrypt-certbot.html
https://segmentfault.com/a/1190000005797776
http://www.tuicool.com/articles/ZZrAN32
2.官方文档
https://certbot.eff.org/#centos6-nginx
https://certbot.eff.org/docs/using.html#webroot
还有个坑，网站要开启允许目录访问。