Documentation: add production users doc

Add the first production user: discovery.etcd.io. We should encourage
the commnuity to start adding their stories to this doc.

Documentation/production-users.md

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+# Production Users
+
+This document tracks people and use cases for etcd in production. By creating a list of production use cases we hope to build a community of advisors that we can reach out to with experience using various etcd applications, operation environments, and cluster sizes. The etcd development team may reach out periodically to check-in on your experience and update this list.
+
+## discovery.etcd.io
+
+- *Application*: https://github.com/coreos/discovery.etcd.io
+- *Launched*: Feb. 2014
+- *Cluster Size*: 5 members, 5 discovery proxies
+- *Order of Data Size*: 100s of Megabytes
+- *Operator*: CoreOS, brandon.philips@coreos.com
+- *Environment*: AWS
+- *Backups*: Periodic async to S3
+
+discovery.etcd.io is the longest continuously running etcd backed service that we know about. It is the basis of automatic cluster bootstrap and was launched in Feb. 2014: https://coreos.com/blog/etcd-0.3.0-released/.