Add Flathub as third party repository (helix-editor#6994)

book/src/install.md

@@ -8,6 +8,7 @@
   - [Fedora/RHEL](#fedorarhel)
   - [Arch Linux community](#arch-linux-community)
   - [NixOS](#nixos)
+  - [Flatpak](#flatpak)
   - [AppImage](#appimage)
 - [macOS](#macos)
   - [Homebrew Core](#homebrew-core)
@@ -18,6 +19,9 @@
   - [MSYS2](#msys2)
 - [Building from source](#building-from-source)
   - [Configuring Helix's runtime files](#configuring-helixs-runtime-files)
+    - [Linux and macOS](#linux-and-macos)
+    - [Windows](#windows)
+    - [Multiple runtime directories](#multiple-runtime-directories)
   - [Validating the installation](#validating-the-installation)
   - [Configure the desktop shortcut](#configure-the-desktop-shortcut)
 <!--toc:end-->
@@ -88,6 +92,15 @@ If you are using a version of Nix without flakes enabled,
 [install Cachix CLI](https://docs.cachix.org/installation) and use
 `cachix use helix` to configure Nix to use cached outputs when possible.
 
+### Flatpak
+
+Helix is available on [Flathub](https://flathub.org/en-GB/apps/com.helix_editor.Helix):
+
+```sh
+flatpak install flathub com.helix_editor.Helix
+flatpak run com.helix_editor.Helix
+```
+
 ### AppImage
 
 Install Helix using the Linux [AppImage](https://appimage.org/) format.