Add route53 plugin (#1390)

* Update vendor

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add route53 plugin

This fix adds route53 plugin so that it is possible to
query route53 record through CoreDNS.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/route53/README.md

@@ -0,0 +1,46 @@
+# route53
+
+## Name
+
+*route53* - enables serving zone data from AWS route53.
+
+## Description
+
+The hosts plugin is useful for serving zones from resource record sets in AWS route53.
+This plugin only supports A and AAAA records. The route53 plugin can be used when
+coredns is deployed on AWS.
+
+## Syntax
+
+~~~ txt
+route53 [ZONE:HOSTED_ZONE_ID...] {
+    [aws_access_key AWS_ACCESS_KEY_ID AWS_SECRET_ACCESS_KEY]
+}
+~~~
+
+* **ZONE** the name of the domain to be accessed.
+* **HOSTED_ZONE_ID** the ID of the hosted zone that contains the resource record sets to be accessed.
+* **AWS_ACCESS_KEY_ID** and **AWS_SECRET_ACCESS_KEY** the AWS access key ID and secret access key
+   to be used when query AWS (optional).  If they are not provided, then coredns tries to access
+   AWS credentials the same way as AWS CLI, e.g., environmental variables, AWS credentials file,
+   instance profile credentials, etc.
+
+## Examples
+
+Enable route53, with implicit aws credentials:
+
+~~~ txt
+. {
+    route53 example.org.:Z1Z2Z3Z4DZ5Z6Z7
+}
+~~~
+
+Enable route53, with explicit aws credentials:
+
+~~~ txt
+. {
+    route53 example.org.:Z1Z2Z3Z4DZ5Z6Z7 {
+      aws_access_key AWS_ACCESS_KEY_ID AWS_SECRET_ACCESS_KEY
+  }
+}
+~~~