Add digestLength example

Author: filip26

README.md

@@ -109,9 +109,15 @@ if (MultihashCodec.SHA2_384.isEncoded(encoded)) {
   ...
 }
 
+
 /* get registry initialized with all multihash codecs */
 var registry = MulticodecRegistry.getInstance(Tag.Multihash);
 
+/* determine digest index  */
+var index = registry.getCodec(encoded)
+                    .mapToInt(codec -> codec.length() + codec.digestLength(encoded))
+                    .orElseThrow(() -> new IllegalArgumentException("Unsupported multihash."));
+
 /* encode an input as multihash */
 byte[] encoded = registry.getCodec(code)
                          .map(codec -> codec.encode(input))