While running the following, noticed the whitespace warning from presubmit:
```
ubuntu@ubuntu:~/coredns$ docker run -i -t --rm -v $PWD:/go/src/github.com/coredns/coredns --net=host golang:1.11
root@ubuntu:/go# cd src/github.com/coredns/coredns/
root@ubuntu:/go/src/github.com/coredns/coredns# make
** presubmit/context
** presubmit/filename-hyphen
** presubmit/test-lowercase
** presubmit/trailing-whitespace
plugin/auto/README.md:* `reload` interval to perform reload of zone if SOA version changes. Default is one minute.
plugin/auto/README.md:  Value of `0` means to not scan for changes and reload. eg. `30s` checks zonefile every 30 seconds
plugin/file/README.md:* `reload` interval to perform reload of zone if SOA version changes. Default is one minute.
plugin/file/README.md:  Value of `0` means to not scan for changes and reload. eg. `30s` checks zonefile every 30 seconds
** presubmit/trailing-whitespace: please remove any trailing white space
```
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>