* plugin/etcd: clarify it's not a generic backend

Clarify the docs that this is not a generic backend.

Wrap it on 80 columns as well.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Update plugin/etcd/README.md
Co-Authored-By: Chris O'Haver <cohaver@infoblox.com>

* Update plugin/etcd/README.md
Co-Authored-By: Chris O'Haver <cohaver@infoblox.com>