contributing: add CONTRIBUTING file

Author: Kátia Nakamura

CONTRIBUTING.md

@@ -0,0 +1,15 @@
+==========================================================================
+
+Contributing to PyCon Balkan website
+==========================================================================
+
+PyCon Balkan website is an open source project and welcomes contributions.
+
+To start contributing, fork/clone the project, install all the requirements and create a file named 
+`.env` in the project's root.
+
+Inside of the `.env` file added:
+```
+SECRET_KEY={create_and_add_your_own_SECRET_KEY_here_with_no_spaces}
+DEBUG=True
+```