Add description of semantic versioning for OnSIDES

[ntatonetti]

We are roughly following the recommendations on semantic versioning (see https://semver.org) of Major.Minor.Patch. Major versions are those that change the database structure (~API structure), Minor are significant improvements to the pipeline that may alter the database entires (ADR extractions), and Patch are minor improvements and bug fixes that will not affect the database entries.

Add a VERSIONING.md file that explains this with good formatting.