update document about role-arn

README.md

@@ -46,6 +46,16 @@ You can switch between different sets in the credentials file by passing
 
         cli53 list --profile my_profile
 
+You can also assume a specific role by passing `--role-arn` to any command.
+For example:
+
+        cli53 list --role-arn arn:aws:iam::123456789012:role/myRole
+
+You can combine role with profile.
+For example:
+
+        cli53 list --profile my_profile --role-arn arn:aws:iam::123456789012:role/myRole
+
 For more information, see: http://blogs.aws.amazon.com/security/post/Tx3D6U6WSFGOK2H/A-New-and-Standardized-Way-to-Manage-Credentials-in-the-AWS-SDKs
 
 Note: for Alpine on Docker, the pre-built binaries do not work, so either use Debian, or follow the instructions below for Building from source.