add CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute to the project
+
+Thank you very much for you interest in the project! :smile:
+
+
+First, choose a issue (or create one with suggestions, ideas, tips...) and wait for the assignment.
+
+Then clone the repository and work on the development branch. 
+
+## Setup
+
+We strongest suggest create a virtualenv, activate and install requirements.
+
+ $ python3 -m venv env
+ $ source env/bin/activate
+ (env) $ pip install -r requirements-dev.txt
+
+## Run tests
+
+Get into the virtualenv, and:
+
+ (env) $ python -m pytest tests/