[DOCS] Updates CHANGELOG for 9.0.1

Author: picandocodigo

CHANGELOG.md

@@ -1,5 +1,9 @@
 **See the full release notes on the official documentation website: https://www.elastic.co/docs/release-notes/elasticsearch/clients/ruby**
 
+# 9.0.1
+
+- The request headers were updated for Elasticsearch v9: `compatible-with=9` [#2660](https://github.com/elastic/elasticsearch-ruby/pull/2660).
+
 # 9.0.0
 
 Ruby 3.2 and up are tested and supported for 9.0. Older versions of Ruby have reached their end of life. We follow Ruby’s own maintenance policy and officially support all currently maintained versions per [Ruby Maintenance Branches](https://www.ruby-lang.org/en/downloads/branches/). The required Ruby version is set to `2.6` to keep compatiblity wit JRuby 9.3. However, we only test the code against currently supported Ruby versions.