added merge by id description

docs/data-provision/merge-by-id.md

@@ -1,3 +1,28 @@
 # Merge by id
 
-<span className="todo">TODO</span>
+In the metadata aggregation system it is common to find the same record provided by 
+different datasources and, sometimes, even inside the same datasource (especially in 
+case of aggregators). As the harmonisation processes are performed per datasource 
+contents, the relative records are the output of different mapping implementations. 
+This approach has the advantage to be deeply customisable to catch datasource specific 
+aspects, but it leaves room for inconsistencies when evaluating the different mappings 
+across the various datasources.
+
+This phase is therefore responsible to compensate for such inconsistencies and performs
+a global grouping of every record available in the graph:
+
+- entities are grouped by [`id`](../data-model/entities/result#id)
+- relations are grouped by [`source`, `target`, `reltype`](../data-model/relationships#the-relationship-object)
+
+This ensures that the same record, possibly assigned to different types by different 
+mappings, appears only once in the graph and under a single typing. In case of clashing 
+identifiers, the properties are merged (including the provencance information), considering 
+the following precedence order for the result typing:
+
+```
+publication > dataset > software > other
+```
+
+The same holds for relationships, as the same (e.g.) DOI-to-DOI citation relation could 
+be aggregated from multiple sources, this grouping phase would collapse all the different 
+duplicates onto a single relation that would however include all the individual provenances.