[fractal/contributing] Add contribution guidelines

[Alexandre Franke <afranke src gnome org>]

commit e2e82882528ace1020dc421d843aa9372b491f0d
Author: Alexandre Franke <afranke gnome org>
Date:   Sat Feb 13 21:18:09 2021 +0100

    Add contribution guidelines
    
    Fixes #722

 CONTRIBUTING.md | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)
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diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
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index 00000000..97425020
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+# 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.