Fixed a typo in README
Merge branch 'master' of https://github.com/valyala/ybc
go-cdn-booster improvements:
1) pass Last-Modified response header, so http clients and proxies could send If-Modified-Since requests.
2) pass Connection: keep-alive response header, so http clients could re-use the same connection to go-cdn-booster.
3) pass Server: go-cdn-booster response header, so http clients could be aware of server software.