docs: Create CONTRIBUTING.md (#2416)

Co-authored-by: Pepe Fagoaga <pepe@verica.io>

CONTRIBUTING.md

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+# Do you want to learn on how to...
+
+- Contribute with your code or fixes to Prowler
+- Create a new check for a provider
+- Create a new security compliance framework
+- Add a custom output format
+- Add a new integration
+- Contribute with documentation
+
+Want some swag as appreciation for your contribution?
+
+# Prowler Developer Guide
+https://docs.prowler.cloud/en/latest/tutorials/developer-guide/