btrfs-progs: docs: add 6.15 kernel development statistics

[ci skip]

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

Documentation/Contributors.rst

@@ -59,6 +59,7 @@ Statistics for 6.x series
     "6.12", "20", "111881", "163548", "148", "+1868 -1804"
     "6.13", "25", "112756", "164722", "156", "+2780 -1579"
     "6.14", "20", "114592", "167178", "147", "+3847 -1391"
+    "6.15", "25", "114616", "167485", "190", "+2346 -2039"
 
 
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