Add changelog for v5

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 at the level of the [result instance](/data-model/entities/result#instance)
+
+- New [relationship types](/data-model/relationships#relationship-types): `Reviews, IsReviewedBy, IsParentOf, IsChildOf, References, IsReferencedBy, IsNewVersionOf, IsPreviousVersionOf, HasVersion, IsVersionOf, Cites, IsCitedBy, IsVariantFormOf, IsOriginalFormOf, Obsoletes, IsObsoletedBy, IsContinuedBy, Continues, Documents, IsDocumentedBy, IsCompiledBy, Compiles, IsPartOf, HasPart, IsSourceOf, IsDerivedFrom, IsIdenticalTo`
+
+#### Changed
+
+- FOS and SDGs were removed from the [result subjects](/data-model/entities/result#subjects)
+