Update api README

Kubernetes-commit: 2b237717a851329a3861fac0f020f2ab96409edf

README.md

@@ -1 +1,19 @@
-This repo is still in the experimental stage. Shortly it will contain the schema of the API that are served by the Kubernetes apiserver.
+# api
+
+Schema of the external API types that are served by the Kubernetes API server.
+
+## Purpose
+
+This library is the canonical location of the Kubernetes API definition. Most likely interaction with this repository is as a dependency of client-go.
+
+## Compatibility
+
+Branches track Kubernetes branches and are compatible with that repo.
+
+## Where does it come from?
+
+`api` is synced from https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/api. Code changes are made in that location, merged into `k8s.io/kubernetes` and later synced here.
+
+## Things you should *NOT* do
+
+1. https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/api is synced to k8s.io/api. All changes must be made in the former. The latter is read-only.