Adding documentation about how to use single-instance plugin with snap and flatpak packages.

Author: charlesschaefer

src/content/docs/plugin/single-instance.mdx

@@ -122,3 +122,75 @@ pub fn run() {
         .expect("error while running tauri application");
 }
 ```
+
+## Working with Snap and Flatpak
+
+The Single Instance plugin uses DBus to ensure it will be only one instance running. It does that by publishing a service to DBus when the first instance starts running.
+Then, the following instances will try to publish the same service and, if it is already published, they will send a request to the service to notify the first instance, and finish right away.
+
+Despite this works pretty well when your app is bundled as a deb or rpm package, it won't work as intended for snap or flatpak packages because these packages run in a constrained sandboxed environment, where most of the communication to DBus services will be blocked if not explicitly declared on the packaging manifest.
+
+Here's a guide to declare the needed permissions to enable the Single Instance for snap and flatpak packages:
+
+### Getting your app ID
+
+The Single Instance plugin will publish a service named `org.{id}.SingleInstance`. 
+
+`{id}` will be the key **identifier** from your **tauri.conf.json** file, but with with dots (.) and dashes (-) replaced by underline (_).
+
+So, if your identifier is `net.mydomain.MyApp`:
+- `net_mydomain_MyApp` will be your app `{id}`
+- `org.net_mydomain_MyApp.SingleInstance` will be your app SingleInstance service name
+
+You will need the service name to authorize your app to use the DBus service on snap and flatpak manifests, as seen below. 
+
+### Snap 
+
+On your snapcraft.yml file, declare a plug and a slot for the single instance service, and use both on your app declaration:
+```yaml title="snapcraft.yml"
+# ...
+slots:
+  single-instance:
+    interface: dbus
+    bus: session
+    name: org.net_mydomain_MyApp.SingleInstance # Remember to change net_mydomain_MyApp to your app ID
+
+plugs:
+  single-instance-plug:
+    interface: dbus
+    bus: session
+    name: org.net_mydomain_MyApp.SingleInstance # Remember to change net_mydomain_MyApp to your app ID
+
+# .....
+apps:
+  my-app:
+    # ...
+    plugs:
+      # ....
+      - single-instance-plug
+    slots:
+      # ...
+      - single-instance
+    
+    # ....
+```
+
+This will allow your app to listen and to send requests to the DBus service as expected by the Single Instance plugin.
+
+### Flatpak
+
+On your flatpak manifest file (your.app.id.yml or your.app.id.json), declare a `--talk-name` and a `--own-name` finish args with the service name:
+```yaml title="net.mydomain.MyApp.yml"
+# ...
+finish-args:
+  - --socket=wayland
+  - --socket=fallback-x11
+  - --device=dri
+  - --share=ipc
+  # ....
+  - --talk-name=org.net_mydomain_MyApp.SingleInstance # Remember to change net_mydomain_MyApp to your app ID
+  - --own-name=org.net_mydomain_MyApp.SingleInstance # Remember to change net_mydomain_MyApp to your app ID
+# ...
+```
+
+This will allow your app to listen and to send requests to the DBus service as expected by the Single Instance plugin.