Update development instructions (#154)

… based on an inquiry from @s-zeng in #152 (comment)

Author: Gabriella439

README.md

@@ -306,6 +306,24 @@ in
 
 ## Development
 
+The logic for generating the Dhall code doesn't reside within this
+repository but actually resides within the
+[`dhall-openapi`](https://github.com/dhall-lang/dhall-haskell/tree/master/dhall-openapi)
+subproject of the [`dhall-haskell`](https://github.com/dhall-lang/dhall-haskell)
+repository.  That means that if you want to change the generated code you will
+need to do so in two steps:
+
+* Make a pull request against the upstream `dhall-haskell` repository to
+  change the code generated by `dhall-openapi`
+
+* Make a pull request against this repository to pick up a newer reference
+  to the `dhall-haskell` repository incorporating the change to `dhall-openapi`
+
+If you try to create a pull request to amend the generated Dhall files directly
+then CI will reject the pull request since it verifies that the Dhall code
+stored in version control matches what `dhall-openapi` would generate from the
+Kubernetes OpenAPI specification.
+
 ### Updating the `dhall-openapi` dependency
 
 The `dhall-openapi` dependency is a subproject of the `dhall-haskell`