Document how to use EU domains (mailgun#175)

Author: dcabo

README.md

@@ -56,6 +56,12 @@ domain = 'example.com'
 result = mg_client.get("#{domain}/events", {:event => 'delivered'})
 ```
 
+If you're using the EU domains, make sure you specify it when creating the client:
+
+```
+mg_client = Mailgun::Client.new 'your-api-key', 'api.eu.mailgun.net'
+```
+
 Rails
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