Resolve "Add publiccode.yml file"

Closes #113 (closed)

This MR adds a publiccode.yml file. This publiccode.yml file could be used by aggregators like code.gouv.fr or developers.italia.it to extract some informations associated to this repository.

• project: https://github.com/publiccodeyml/publiccode.yml
• documentation: https://docs.italia.it/italia/developers-italia/publiccodeyml-en/en/master/index.html
• online generator:
  • https://publiccode-editor.developers.italia.it/
  • https://publiccode-editor.etalab.studio/
• dedicated search engine for Italy: https://developers.italia.it/en/search
• example:
  • https://github.com/globaleaks/GlobaLeaks/blob/main/publiccode.yml
  • https://github.com/tdf/libreoffice-metadata/blob/master/publiccode.yml
• Parser (Go) https://github.com/italia/publiccode-parser-go
• Validator (Go) : https://github.com/italia/publiccode-validator
• IDE validator (IntelliJ IDEA, VS Code) via https://json.schemastore.org/publiccode.json
• CI validator: https://github.com/megalinter/megalinter (not tested)

  CI/CD applications can detect all JSON and YAML files and validate them if a matching schema is found on SchemaStore.org