Docs: Add note about flatpak manifests

Fixes #60.

docs/flatpak.rst

@@ -0,0 +1,25 @@
+=======
+Flatpak
+=======
+
+Flathub's builders don't allow internet access during the build; everything that
+goes into the build must be specified in the manifest. This means meson
+submodules won't work. Instead, you need to install blueprint-compiler as
+a module in your flatpak manifest:
+
+.. code-block:: json
+
+   {
+     "name": "blueprint-compiler",
+     "buildsystem": "meson",
+     "sources": [
+       {
+         "type": "git",
+         "url": "https://gitlab.gnome.org/jwestman/blueprint-compiler",
+         "branch": "main"
+       }
+     ]
+   }
+
+You can keep the submodule configuration--Meson will ignore it if
+blueprint-compiler is already installed.