AC-6665: Added Readme file for ES8 module

Author: faizan-shk

app/code/Magento/Elasticsearch8/README.md

@@ -0,0 +1,32 @@
+#Magento_Elasticsearch8 module
+
+Magento_Elasticsearch8 module allows using ElasticSearch engine 8.x version for the product searching capabilities.
+
+The module implements Magento_Search library interfaces.
+
+## Installation details
+
+The Magento_Elasticsearch8 module is one of the base Magento 2 modules. Disabling or uninstalling this module is not recommended.
+
+For information about a module installation in Magento 2, see [Enable or disable modules](https://experienceleague.adobe.com/docs/commerce-operations/installation-guide/tutorials/manage-modules.html).
+
+## Structure
+
+`SearchAdapter/` - the directory that contains solutions for adapting ElasticSearch query searching.
+
+For information about a typical file structure of a module in Magento 2, see [Module file structure](https://devdocs.magento.com/guides/v2.4/extension-dev-guide/build/module-file-structure.html#module-file-structure).
+
+## Additional information
+
+By default`indices.id_field_data`  is disallowed in Elasticsearch8 hence it needs to enabled it from `elasticsearch.yml`
+by adding the following configuration
+`indices:
+id_field_data:
+enabled: true`
+
+More information about ElasticSearch are at articles:
+
+- [Configuring Catalog Search](https://experienceleague.adobe.com/docs/commerce-admin/catalog/catalog/search/search-configuration.html).
+- [Installation Guide/Elasticsearch](https://experienceleague.adobe.com/docs/commerce-operations/installation-guide/prerequisites/search-engine/overview.html).
+- [Configure and maintain Elasticsearch](https://experienceleague.adobe.com/docs/commerce-operations/configuration-guide/search/overview-search.html).
+- Magento Commerce Cloud - [set up Elasticsearch service](https://devdocs.magento.com/cloud/project/services-elastic.html).