Add donation info to README.

README.md

@@ -92,3 +92,7 @@ Some older versions of PJSIP are affected (confirmed for 2.3). In this case you
 ## Credits
 
 2015, 2016 Michał Słomkowski. The code is published under the terms of Apache License 2.0.
+
+## Donations
+
+If this project has helped you and you feel generous, you can donate some money to `14qNqXwqb6zsEKZ6vUhWVbuNLGdg8hnk8b`.