video/gtk4: Add a flatpak snippet example in the README

Close #155

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1102>

video/gtk4/README.md

@@ -5,3 +5,57 @@ offer the possibility to use a custom `gst::Pipeline`. The plugin provides a `gs
 
 The Sink can generate GL Textures if the system is capable of it, but it needs to be compiled
 with either `wayland`, `x11glx` or `x11egl` cargo features.
+
+# Flatpak Integration
+
+To build and include the plugin in a Flatpak manifest, you can add the following snippet to your json manifest:
+
+```json
+{
+    "sdk-extensions": [
+        "org.freedesktop.Sdk.Extension.rust-stable"
+    ],
+    "build-options": {
+        "env": {
+            "CARGO_HOME": "/run/build/cargo-c/cargo"
+        },
+        "append-path": "/usr/lib/sdk/rust-stable/bin",
+    },
+    "modules": [
+        {
+            "name": "cargo-c",
+            "buildsystem": "simple",
+            "build-commands": [
+                "cargo install cargo-c --root /app"
+            ],
+            "build-options": {
+                "build-args": [
+                    "--share=network"
+                ]
+            },
+            "cleanup": [
+                "*"
+            ]
+        },
+        {
+            "name": "gst-plugins-rs",
+            "buildsystem": "simple",
+            "sources": [
+                {
+                    "type": "git",
+                    "url": "https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs",
+                    "branch": "0.10"
+                }
+            ],
+            "build-options": {
+                "build-args": [
+                    "--share=network"
+                ]
+            },
+            "build-commands": [
+                "cargo cinstall -p gst-plugin-gtk4 --prefix=/app"
+            ]
+        }
+    ]
+}
+```