Add "pull" command in acloud README.md.

Bug: 120613398
Test: acloud --help
Change-Id: I96b7cc8009e878beeb2a6b6f8046c43fab9dce13
diff --git a/public/acloud_main.py b/public/acloud_main.py
index fb69e5e..65e649b 100644
--- a/public/acloud_main.py
+++ b/public/acloud_main.py
@@ -56,6 +56,13 @@
    To show more detail info on the list.
    $ acloud list -vv
 
+-  Pull:
+   Pull will download log files or show the log file in screen from one remote
+   cuttlefish instance:
+   $ acloud pull
+   Pull from a specified instance:
+   $ acloud pull --instance-name "your_instance_name"
+
 Try $acloud [cmd] --help for further details.
 
 """