flatpak: Add flatpak manifest

This is intended to be used for easier hacking, similarly to Settings.

gnome-user-docs and gnome-getting-started-docs aren't strictly required
for building, but they provide content to be displayed.

org.gnome.Yelp.json

+{
+    "app-id" : "org.gnome.Yelp",
+    "runtime" : "org.gnome.Platform",
+    "runtime-version" : "master",
+    "sdk" : "org.gnome.Sdk",
+    "rename-desktop-file" : "yelp.desktop",
+    "command" : "yelp",
+    "tags" : [
+        "devel",
+        "development",
+        "nightly"
+    ],
+    "desktop-file-name-suffix" : " (Development)",
+    "finish-args" : [
+        /* X11 + XShm access */
+        "--share=ipc",
+        "--socket=x11",
+        /* Wayland access */
+        "--socket=wayland",
+        /* Needed for dconf to work */
+        "--filesystem=xdg-run/dconf",
+        "--filesystem=~/.config/dconf:ro",
+        "--talk-name=ca.desrt.dconf",
+        "--env=DCONF_USER_CONFIG_DIR=.config/dconf"
+    ],
+    "modules" : [
+        {
+            "name" : "gnome-user-docs",
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://gitlab.gnome.org/GNOME/gnome-user-docs.git"
+                }
+            ]
+        },
+        {
+            "name" : "gnome-getting-started-docs",
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://gitlab.gnome.org/GNOME/gnome-getting-started-docs.git"
+                }
+            ]
+        },
+        {
+            "name" : "yelp",
+            "builddir" : true,
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://gitlab.gnome.org/GNOME/yelp.git"
+                }
+            ]
+        }
+    ]
+}