Merge pull request #34 from geomars/master

Add USAGE.md

Author: Matt Bernier

README.md

@@ -46,7 +46,7 @@ pip install smtpapi
 from smtpapi import SMTPAPIHeader
 header = SMTPAPIHeader()
 header.add_to('[email protected]')
-print header.json_string()
+print(header.json_string())
 ```
 
 <a name="usage"></a>