Add docs about using elasticsearch as a storage driver

[nik-kor]

It's strange that there is not docs about ES storage driver. It would be useful for me.

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[timstclair]

Capitalize the "search" in ElasticSearch, same below.

[timstclair]

Thanks! I've been meaning to separate the storage documentation from the main readme, but this is ok for now.

[timstclair]

ok to test

[k8s-bot]

Jenkins GCE e2e

Build/test passed for commit 20f40eb.

• Build Log

[nik-kor]

I signed it!

[googlebot]

CLAs look good, thanks!

[k8s-bot]

Jenkins GCE e2e

Build/test passed for commit 82c690c.

• Build Log

[timstclair]

Can you reword this to something like, ElasticSearch can use a sniffing process to find all nodes of your cluster automatically. False by default.

[timstclair]

One last comment, then LGTM.

[nik-kor]

done. rebased it

[k8s-bot]

Jenkins GCE e2e

Build/test passed for commit f2beaa5.

• Build Log

[timstclair]

Thanks! LGTM