Add openAPI specifications for cases endpoint

x-pack/plugins/cases/docs/openapi/README.md

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+# OpenAPI (Experimental)
+
+The current self-contained spec file is [as JSON](https://raw.githubusercontent.com/elastic/kibana/master/x-pack/plugins/cases/common/openapi/bundled.json) or [as YAML](https://raw.githubusercontent.com/elastic/kibana/master/x-pack/plugins/cases/common/openapi/bundled.yaml) and can be used for online tools like those found at https://openapi.tools/. 
+This spec is experimental and may be incomplete or change later.
+
+A guide about the openApi specification can be found at [https://swagger.io/docs/specification/about/](https://swagger.io/docs/specification/about/).
+
+## The `openapi` folder
+
+* `entrypoint.yaml` is the overview file which pulls together all the paths and components.
+* [Paths](paths/README.md): this defines each endpoint.  A path can have one operation per http method.
+* [Components](components/README.md): Reusable components
+
+## Tools
+
+It is possible to validate the docs before bundling them with the following
+command in the `x-pack/plugins/cases/docs/openapi/` folder:
+
+  ```
+    npx swagger-cli validate entrypoint.yaml
+  ```
+
+Then you can generate the `bundled` files by running the following commands:
+
+    ```
+    npx @redocly/openapi-cli bundle --ext yaml --output bundled.yaml entrypoint.yaml
+    npx @redocly/openapi-cli bundle --ext json --output bundled.json entrypoint.yaml
+    ```
+