Add instructions to install using FlatPak

After LibrePCB/librepcb-rfcs#26, LibrePCB can now be installed with
FlatPak.

This change adds basic instructions, assuming users already have FlatPak
installed in their machines. Links to FlatPak setup guide are provided as
convenience, since instructions vary on platform.

Fixes LibrePCB#21

getting_started/installation.adoc

@@ -54,6 +54,17 @@ chmod +x ./{linux-installer-filename}
 ./{linux-installer-filename}
 ----
 
+LibrePCB is also available as a https://flatpak.org[FlatPak] package from
+https://flathub.org/apps/details/org.librepcb.LibrePCB[FlatHub]. Assuming you
+have followed https://flatpak.org/setup/[FlatPak setup], you can configure
+FlatHub and install LibrePCB as follows:
+
+[source,bash,subs="attributes"]
+----
+flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
+flatpak install flathub org.librepcb.LibrePCB
+----
+
 [discrete]
 === Mac
 :mac-installer-filename: librepcb-installer-{version}-mac-x86_64.dmg