added install instruction with apt

Closes #6206

Author: clintongormley

docs/reference/setup/repositories.asciidoc

@@ -23,7 +23,13 @@ Add the following to your /etc/apt/sources.list to enable the repository
 deb http://packages.elasticsearch.org/elasticsearch/1.1/debian stable main
 --------------------------------------------------
 
-Run apt-get update and the repository is ready for use.
+Run apt-get update and the repository is ready for use. You can install it with :
+
+[source,sh]
+--------------------------------------------------
+apt-get install elasticsearch
+--------------------------------------------------
+
 
 
 [float]