Document that package sources are ignored by Pip

Author: smheidrich

docs/repositories.md

@@ -119,6 +119,18 @@ when publishing a package.
 
 {{% /note %}}
 
+{{% warning %}}
+
+Package sources are a Poetry-specific feature and **not** included in
+[core metadata](https://packaging.python.org/en/latest/specifications/core-metadata/) produced by
+the Poetry build backend.
+
+Consequently, when a Poetry project is e.g. installed using Pip (as a normal package or in editable
+mode), package sources will be ignored and the dependencies in question downloaded from PyPI by
+default.
+
+{{% /warning %}}
+
 ### Project Configuration
 
 These package sources may be managed using the [`source`]({{< relref "cli#source" >}}) command for