Add development notes

docs/Dev.md

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+## Monorepo
+
+The monorepo uses Yarn (classic) workspaces.
+
+To run a command for a workspace `<ws>`, invoke `yarn workspace <ws> <cmd>` from
+the root folder instead the the `yarn <cmd>` you’d have done otherwise.
+
+To add a local package as a dependency, use `<package-name>@*`. The "*" here
+denotes any version.
+
+```sh
+yarn workspace photos add '@/utils@*'
+```
+
+To see what packages depend on each other locally, use
+
+```sh
+yarn workspaces info
+```

docs/README.md

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+## Notes for maintainers
+
+If you just want to run ente locally or develop on it, you can do
+
+    yarn
+    yarn photos:dev
+
+The notes in this directory are for maintainers to note down more advanced or
+infrequently needed details.