readme: add info about contributing and trello

Author: Kátia Nakamura

README.md

@@ -1,3 +1,7 @@
 # pyconbalkan [![Build Status](https://travis-ci.org/PythonBalkan/pyconbalkan.svg?branch=master)](https://travis-ci.org/PythonBalkan/pyconbalkan)
 
 Website for PyCon Balkan
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+> Info about contributing: https://github.com/PythonBalkan/pyconbalkan/blob/master/CONTRIBUTING.md
+
+> Public Trello Board: https://trello.com/b/J6NhX1GZ/pycon-balkan-2018