Describe versioning principles

mkdocs.yml

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 nav:
   # - Home: home.md
   - Index: index.md
+  - Principles: principles.md
   - About: about.md
 site_url: https://nfdi4cat.github.io/pid4cat-model
 repo_url: https://github.com/nfdi4cat/pid4cat-model

src/docs/files/principles.md

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+# Versioning of PID4Cat-model
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+The versioning of the PID4Cat-model is following the **SchemaVer** scheme proposed in a [snowplow blog post](https://snowplow.io/blog/introducing-schemaver-for-semantic-versioning-of-schemas). This also matches with the [HCA schema versioning](https://github.com/HumanCellAtlas/metadata-schema/blob/master/docs/evolution.md#schema-versioning) and is also adapted by [openlinaeage](https://github.com/OpenLineage/OpenLineage/blob/main/spec/Versioning.md).
+
+**SchemaVer**: Given a version number MODEL.REVISION.ADDITION, increment the
+
+- MODEL when you make a breaking schema change which will prevent interaction with any historical data
+- REVISION when you make a schema change which may prevent interaction with some historical data
+- ADDITION when you make a schema change that is compatible with all historical data
+
+The versions are tagged in git (for example "v1.2.3"). 
+Based on the tags we also create GitHub releases to provide easy access to all versions and to document the changes between versions.