add whats new entry

(cherry picked from commit f821030)

Author: rustagir

source/whats-new.txt

@@ -75,6 +75,13 @@ What's New in 1.20
   encryption and decryption of the data key locally, ensuring that the
   encryption key never leaves the KMIP server.
 
+- Adds the ``type`` option in Search index specifications for
+  the :phpmethod:`MongoDB\Collection::createIndex()` and
+  :phpmethod:`MongoDB\Collection::createSearchIndexes()` methods. This
+  change allows you to create Atlas Vector Search indexes
+  programmatically. To learn more and view examples, see the
+  :ref:`php-atlas-search-index` guide.
+
 For more information about the changes in this version, see the
 :github:`v1.20 release notes
 </mongodb/mongo-php-library/releases/tag/1.20.0>` on GitHub.
@@ -135,4 +142,5 @@ What's New in 1.17
   7.2 and 7.3 has been removed and the library now requires PHP 7.4 or newer.
 
 To learn more about this release, see the `v1.17 Release Notes 
-<https://github.com/mongodb/mongo-php-library/releases/tag/1.17.0>`__ on GitHub.
+<https://github.com/mongodb/mongo-php-library/releases/tag/1.17.0>`__ on
+GitHub.