Merge pull request 'Add versioning section & changelog' (#10) from changelog into main

Reviewed-on: D-Net/openaire-graph-docs#10

docs/changelog.md

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-# Changelog
-<span className="todo">TODO</span>
+# Versions & changelog
+
+
+## Versioning
+
+Our versioning policy follows the [Semantic Versioning specification](https://semver.org/). 
+In our case, given a version `MAJOR.MINOR.PATCH`, we increment the:
+
+* `MAJOR` version when the data model of the Graph changes
+* `MINOR` version when the pipeline (e.g., different deduplication method, different implementation for an enrichment process) or major data sources change
+* `PATCH` version when the graph data are updated
+
+
+## Changelog
+
+This section will document all notable changes for each graph version.
+
+
+### v5.0.0
+
+#### Added
+
+- [Impact indicators](/data-model/entities/result#indicators) at the level of the Result
+- [Beginner's kit](/downloads/beginners-kit) in the Downloads section
+- New relation types. See the [complete set of relationship](/data-model/relationships#relationship-types)
+- 
+#### Changed
+
+- FOS and SDGs were removed from the [result subjects](/data-model/entities/result#subjects)
+- Measures were removed from the [result instance](/data-model/entities/result#instance)
+