[moserial: 4/11] Add clone command for building the Flatpak to the README

[Michael J. Chudobiak <mjc src gnome org>]

commit 33c29f6881e2fd790334030aa8de9fe72d828837
Author: Jordan Williams <jordan jwillikers com>
Date:   Fri Jul 9 14:08:12 2021 -0500

    Add clone command for building the Flatpak to the README
    
    There's a Git submodule, so `--recursive` is necessary.

 README | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/README b/README
index a58bb2f..6fcb248 100644
--- a/README
+++ b/README
@@ -18,6 +18,12 @@ For more information see [GNOME wiki](https://wiki.gnome.org/Apps/Moserial).
 
 ### Flatpak
 
+Get the source code.
+```
+git clone --recursive https://gitlab.gnome.org/GNOME/moserial.git
+cd moserial
+```
+
 Add the Flathub repository.
 ```
 flatpak remote-add --user --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo