feat: add doc change

Signed-off-by: SuZhou-Joe <[email protected]>

CHANGELOG.md

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+# CHANGELOG
+
+Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
+
+### 📈 Features/Enhancements
+
+- Add support for registerMessageParser ([#5](https://github.com/opensearch-project/dashboards-assistant/pull/5))

server/README.md

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+# `registerMessageParser` — Register your customized parser logic into Chatbot.
+
+**Interaction** refers to a question-answer pair in Chatbot application. In most cases, an interaction consists of two messages: an `Input` message and an `Output` message. However, as the Chatbot evolves to become more powerful, it may display new messages such as visualizations, data explorers, or data grids. Therefore, it is crucial to implement a mechanism that allows other plugins to register their customized parser logic based on each interaction body.
+
+
+## API
+
+### registerMessageParser
+
+```
+dashboardAssistant.registerMessageParser({
+    id: "foo_parser",
+    parserProvider: async (interaction, messageParserHelper) => {
+        if (interaction.additional_info?.visualizationId) {
+            messageParserHelper.addMessage({
+                contentType: "visualization",
+                content: interaction.additional_info.visualizationId
+            })
+        }
+    }
+})
+```