Remove extend from unused keys, add to conversion doc

This removes the "unsupported" extends from the list (since we actually
support) as well as add a clarificiation on the conversion.md document.

Closes https://github.com/kubernetes-incubator/kompose/issues/475
Closes https://github.com/kubernetes-incubator/kompose/issues/493

pkg/loader/compose/compose.go

@@ -59,7 +59,6 @@ func checkUnsupportedKey(composeProject *project.Project) []string {
 		"DNSSearch":     false,
 		"DomainName":    false,
 		"EnvFile":       false,
-		"Extends":       false,
 		"ExternalLinks": false,
 		"ExtraHosts":    false,
 		"Hostname":      false,