README: warn about repository transfer

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

README.md

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+**WARNING**: etcd will be transferred from https://github.com/coreos to https://github.com/etcd-io, on 5PM PST, Mon, August 27, 2018. See https://github.com/coreos/etcd/issues/9965 for detailed timeline.
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 # etcd
 
 [![Go Report Card](https://goreportcard.com/badge/github.com/coreos/etcd?style=flat-square)](https://goreportcard.com/report/github.com/coreos/etcd)