[librsvg] COMPILING.md - mention that gtk-doc is required

[Federico Mena Quintero <federico src gnome org>]

commit ea6201206ca84ec5672222cfd2e5315476d40208
Author: Federico Mena Quintero <federico gnome org>
Date:   Fri Sep 4 11:27:57 2020 -0500

    COMPILING.md - mention that gtk-doc is required
    
    autogen.sh depends on it.

 COMPILING.md | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/COMPILING.md b/COMPILING.md
index 6e3e80ab..c02fdd62 100644
--- a/COMPILING.md
+++ b/COMPILING.md
@@ -42,6 +42,7 @@ minimum version is listed here; you may use a newer version instead.
 * Gdk-pixbuf 2.20.0
 * GIO 2.24.0
 * GObject-Introspection 0.10.8
+* Gtk-doc 1.13
 * Libxml2 2.9.0
 * Pango 1.38.0