Merge pull request donnemartin#36 from donnemartin/mad01-change-zookeeper-as-sd

Update service discovery section to include consul, etcd, and additional details

Author: donnemartin

README.md

@@ -729,7 +729,7 @@ Pinterest, for example, could have the following microservices: user profile, fo
 
 ### Service Discovery
 
-Systems such as [Zookeeper](http://www.slideshare.net/sauravhaloi/introduction-to-apache-zookeeper) can help services find each other by keeping track of registered names, addresses, ports, etc.
+Systems such as [Consul](https://www.consul.io/docs/index.html), [Etcd](https://coreos.com/etcd/docs/latest), and [Zookeeper](http://www.slideshare.net/sauravhaloi/introduction-to-apache-zookeeper) can help services find each other by keeping track of registered names, addresses, and ports.  [Health checks](https://www.consul.io/intro/getting-started/checks.html) help verify service integrity and are often done using an [HTTP](#hypertext-transfer-protocol-http) endpoint.  Both Consul and Etcd have a built in [key-value store](#key-value-store) that can be useful for storing config values and other shared data.
 
 ### Disadvantage(s): application layer