Add AWS CLI compatibility info to README

Signed-off-by: Sebastien Goasguen <runseb@gmail.com>

This closes #1

README.rst

diff --git a/README.rst b/README.rst
index 076a8f5..e26ea4b 100644
--- a/README.rst
+++ b/README.rst
@@ -60,7 +60,22 @@
 Usage
 -----
 
-Please see the User Guide_.
+**IMPORTANT**: Please note that the current version of ec2stack only supports AWS Signature Version 2 and therefore will NOT work with the current AWS CLI unless you explicitly tell it to use Version 2.  You can set the signature version for your default AWS CLI profile with:
+
+::
+
+    aws configure set default.ec2.signature_version v2
+
+If you are using named profiles then you can set the version for the specific profile with:
+
+::
+
+    aws configure set profile.<your profile name>.ec2.signature_version v2
+
+Both of the above commands will update your *~/.aws/config* file.
+
+
+For usage information please see the User Guide_.
 
 .. _Guide: https://github.com/BroganD1993/ec2stack/wiki/User-Guide
 .. _CloudStack: http://cloudstack.apache.org