Add README for ECS docs

ecs/docs/README.md

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+# Wazuh Common Schema
+
+The Wazuh Common Schema is a derivation of the [Elastic Common Schema](https://www.elastic.co/guide/en/ecs/current/ecs-field-reference.html) (ECS) providing a common data schema for the different central components of Wazuh.
+
+- [agent](./agent.md)
+- [alerts](alerts.md)
+- [command](commands.md)
+- [states-fim](states-fim.md)
+- [states-inventory-hardware](states-inventory-hardware.md)
+- [states-inventory-hotfixes](states-inventory-hotfixes.md)
+- [states-inventory-networks](states-inventory-networks.md)
+- [states-inventory-packages](states-inventory-packages.md)
+- [states-inventory-ports](states-inventory-ports.md)
+- [states-inventory-processes](states-inventory-processes.md)
+- [states-inventory-system](states-inventory-system.md)
+- [states-vulnerabilities](states-vulnerabilities.md)
+
+---
+
+### Useful resources
+For more information and additional resources, please refer to the following links:
+- [ECS schemas repository](https://github.com/elastic/ecs/tree/main/schemas)