Add packaging_apps_multiinstance.md

packaging_apps_multiinstance.md

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+<a class="btn btn-lg btn-default" href="packaging_apps_en">Application packaging</a>
+
+### Multi-instance
+Multi-instance is application capacity to be installed many times.
+
+#### Scripts
+When YunoHost install a second time the application, it give to the script at last argument `id__2` with the application `id` comming from the manifest. The value `n` in `id__n` is incremented at each new instance of the application.
+
+**E.g.** in the roundcube script, database is called `roundcube`, the install directory `roundcube` and the [Nginx configuration](packaging_apps_nginx_conf_en) `roundcube`. This way, the second instance of roundcube will not conflict with the first one, and will be installed in the `roundcube__2` database, in the `roundcube__2`directory, and with the `roundcube__2` Nginx configuration.
+
+Retrieve last variable gived to scripts:
+```bash
+APP=${!#}
+```
+
+#### Manifest
+Set `multi_instance` variable to `true` in the [manifest](packaging_apps_manifest_en):
+```json
+ "multi_instance": "true",
+```