Add CHANGES.txt entry for sparse LiveDocs optimization

Author: salvatorecampagna

lucene/CHANGES.txt

@@ -80,6 +80,8 @@ Optimizations
 
 * GITHUB#15085, GITHUB#15092: Hunspell suggestions: Ensure candidate roots are not worse before updating. (Ilia Permiashkin)
 
+* GITHUB#13084: Implement sparse LiveDocs format to reduce memory usage by up to 8x for segments with <=1% deletions. (Salvatore Campagna)
+
 Bug Fixes
 ---------------------
 * GITHUB#14049: Randomize KNN codec params in RandomCodec. Fixes scalar quantization div-by-zero