README: use the same tagline from github
Just be consistent with the messaging and use of etcd
![etcd Logo](logos/etcd-horizontal-color.png)
-etcd is a distributed, consistent key-value store for shared configuration and service discovery, with a focus on being:
+etcd is a distributed reliable key-value store for the most critical data of a distributed system, with a focus on being:
* *Simple*: well-defined, user-facing API (gRPC)
* *Secure*: automatic TLS with optional client cert authentication
