docs: Update Meson version policy to mention freedesktop SDK

To avoid this happening again:
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3077#note_1601064.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

docs/meson-version.md

@@ -17,7 +17,9 @@ Policy
  * Stable branches of GLib will not change their Meson dependency after the
    first release of that stable series
  * Unstable branches of GLib can bump their Meson dependency if
-   - at least that version of Meson currently available in Debian Testing; or
+   - at least that version of Meson is currently [available in the freedesktop
+     SDK](https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/blob/master/elements/components/meson.bst); *and*
+   - at least that version of Meson is currently available in Debian Testing, or
    - the Python version required by the new Meson dependency is available in
      Debian Stable *and* the oldest currently-supported Ubuntu LTS
  * The version of Meson used by GLib should be pinned and pre-installed in the
@@ -28,3 +30,8 @@ The reasoning behind allowing a version bump if the Python which Meson depends
 on is available in Debian Stable is that it’s [straightforward to install a more
 recent Meson version using
 `pip`](https://mesonbuild.com/Getting-meson.html#installing-meson-with-pip).
+
+The reasoning behind requiring the version of Meson to be available in the
+freedesktop SDK is that it is used to build GLib on nightly pipelines in
+[gnome-build-meta](https://gitlab.gnome.org/GNOME/gnome-build-meta/), which is
+used to test GLib against multiple third party projects.