Add release notes for flatpak_remote conf key

docs/release-notes/flatpak-remote-is-configurable.rst

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+:Type: Flatpak
+:Summary: Remote repository for Flatpaks after deployment are now configurable
+
+:Description:
+    Currently when OSTree installation detects Flatpak repository in the installation media
+    these Flatpaks are deployed and the remote was hardcoded to remote Fedora. This remote
+    is then used for updating the Flatpaks after installation.
+
+    After this change Flatpak remote could be set by ``flatpak_remote`` key in the configuration
+    file.