Update docs for Yarn Berry

docs/linking.md

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+# Developing with linked packages
+
+If you want to make changes to a package that Element Call depends on and see those changes applied in real time, you can create a link to a local copy of the package. Yarn has a command for this (`yarn link`), but it's not recommended to use it as it ends up modifying package.json with details specific to your development environment.
+
+Instead, you can use our little 'linker' plugin. Create a file named `.links.yaml` in the Element Call project directory, listing the names and paths of any dependencies you want to link. For example:
+
+```yaml
+matrix-js-sdk: ../path/to/matrix-js-sdk
+"@vector-im/compound-web": /home/alice/path/to/compound-web
+```
+
+Then run `yarn install`.