Document merging of OpenAPI and x-kusk file

docs/openapi-extension.md

@@ -46,3 +46,70 @@ Likewise, the Path level settings override what's specified at the global level.
 If settings aren't specified at a path or operation level, it will inherit from the layer above. (Operation > Path > Global)
 
 Please review the generator's documentation to see what can be overwritten.
+
+# Merging vanilla OpenAPI yaml file and x-kusk extention
+
+There are situations when you want to keep your OpenAPI file pristine and not add `x-kusk` extention to it.
+E.g. if you generate it during the build each time, or if you have multiple environments that have different x-kusk overrides per env.
+
+You can always add `x-kusks` enabled YAML file with extention keys and merge it with your OpenAPI file.
+The resulting file can be consumed by Kusk for Ingress generation.
+
+For that, you'll need to use [yt](https://mikefarah.gitbook.io/yq) tool.
+
+E.g.
+
+*petstore.yaml*
+
+```yaml
+paths:
+  "/pet":
+    put:
+      ...
+    post:
+      ...
+```
+
+and *x-kusk.yaml* enabled:
+
+```yaml
+x-kusk:
+  disabled: false
+  cors:
+    origins:
+      - http://foo.example
+      - http://bar.example
+    methods:
+      - POST
+      - GET
+      - OPTIONS
+    headers:
+      - Content-Type
+    credentials: true
+    expose_headers:
+      - X-Custom-Header
+    max_age: 86400
+  service:
+    name: petstore
+    port: 80
+  path:
+    base: /petstore/api/v3
+    trim_prefix: /petstore
+paths:
+  "/pet":
+    x-kusk:
+      disabled: false
+    put:
+      x-kusk:
+        disabled: true
+```
+
+Running the tool:
+
+```shell
+yq eval-all 'select(fileIndex == 0) * select(fileIndex == 1)'  x-kusks.yaml petstore.yaml
+```
+
+will produce merged yaml, ready to be used by Kusk. Note, though, that the order of keys in the resulting map can be different.
+
+If you use JSON for your OpenAPI file, the same result can be achieved with [jq](https://stedolan.github.io/jq/).