Add Flatpak.gitignore

Cribbed directly from the Element Flatpak's .gitignore file at
https://github.com/flathub/im.riot.Riot/blob/master/.gitignore, simply
because Element is a pretty popular application.

Note in particular that this covers all directories that will be
created by the Flatpak tutorial's suggested `flatpak-builder` command:
https://docs.flatpak.org/en/latest/first-build.html#build-and-install

`repo` is the OSTree repository that the built Flatpak will be
"published" to for testing, `.flatpak-builder` is an internal build
directory, and `builddir` is another, user-specified build directory.

Author: strugee

community/Linux/Flatpak.gitignore

@@ -0,0 +1,3 @@
+.flatpak-builder
+builddir
+repo