Merge 80a98b3d2a into 32652622a4

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.

README.md

@@ -131,5 +131,3 @@ by automatically forking the project and prompting to send a pull request too.
 
 [CC0-1.0](./LICENSE).
 
-## Test
-This is just a test

community/Linux/Flatpak.gitignore

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