[mod] Add 'How to contribute to Silkaj' presentation

README.md

public/how_to_contribute_to_silkaj/index.html

public/how_to_contribute_to_silkaj/presentation.md

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public/images/GitLab_logo_text.svg

public/images/GraphQL_Logo.svg

public/images/Python-logo-notext.svg

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public/images/duniterpy_pipeline.png

public/images/poetry_pipeline.png

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public/images/silkaj_0.8_milestone.png

public/images/silkaj_pipeline.png