Add contribution guidelines

Fixes #722

CONTRIBUTING.md

@@ -0,0 +1,18 @@
+# Contributing
+
+## Workflow
+
+[Fractal](https://gitlab.gnome.org/GNOME/fractal/) follows the [GNOME Newcomers workflow](https://wiki.gnome.org/Newcomers/).
+Follow these pages to learn how to contribute.
+
+We only support building with [flatpak](https://flatpak.org/) using [GNOME Builder](https://wiki.gnome.org/Apps/Builder).
+If you insist on using any other method, they may work, but you will be on your own.
+
+You will need to manually install the latest version of the Rust extension of the FreeDesktop SDK.
+To do this, just run `flatpak install --user org.freedesktop.Sdk.Extension.rust-stable//20.08`
+
+## Merge requests
+
+We expect all code contributions to be correctly formatted.
+Before submitting a merge request, please run `cargo fmt` on your branch to ensure this is the case.
+It is also recommended to run `cargo clippy` as that will catch common errors and improve the quality of your submissions.