Add benchmark to measure performance of CustomBinaryDocValuesField

Benchmarks show that ArrayList performs better than TreeSet. Added a comment
on where to find the results and benchmark.

Signed-off-by: Kiran Reddy <kkreddy@amazon.com>

server/src/main/java/org/opensearch/index/mapper/BinaryFieldMapper.java

@@ -242,6 +242,10 @@ protected String contentType() {
      */
     public static class CustomBinaryDocValuesField extends CustomDocValuesField {
 
+        // We considered using a TreeSet instead of an ArrayList here.
+        // Benchmarks show that ArrayList performs much better
+        // For details, see: https://github.com/opensearch-project/OpenSearch/pull/9426
+        // Benchmarks are in CustomBinaryDocValuesFiledBenchmark
         private final ArrayList<byte[]> bytesList;
 
         public CustomBinaryDocValuesField(String name, byte[] bytes) {