Prepare changelog for 9.4.0

Author: kelson42

ChangeLog

@@ -1,3 +1,14 @@
+libzim 9.4.0
+============
+
+ * Add support of Illustrations scale (dpr), height and width (@veloman-yunkan #997)
+ * Many CI updates (@kelson42 #976 #995 #980, @veloman-yunkan #974 #987, @tusharpm #910)
+ * Improve logging primitites (@veloman-yunkan #975 #984)
+ * New ability to preload Xapian indexes (@mgautierfr #958)
+ * New user/dev defined cache sizes (@veloman-yunkan #982)
+ * Centralize cache memory usage (@veloman-yunkan #960)
+ * Stop releasing for PPA Ubuntu Focal 20.04 (@kelson42 #1009)
+
 libzim 9.3.0
 ============