Create Istio API Guidelines (#320)

* Create Istio API Guidelines

* Rename STYLE.md -> STYLE-GUIDE.md and address review comments.

* Polishing English wording

README.md

@@ -12,6 +12,12 @@ repository. This repository *will not* depend on any other repos
 All components of an Istio installation operate on a shared vocabulary of attributes,
 as defined in this repo.
 
+## Style guide
+
+When designing proto-based APIs in the Istio project, please follow the
+[Istio API Style Guide](./STYLE-GUIDE.md).
+
 ## Updating
+
 After the [protobuf](https://github.com/google/protobuf) definitions are updated, the corresponding `*pb.go` files must be generated by running `scripts/generate-protos.sh` and submitted as part of the same PR as the updated definitions.
 

STYLE-GUIDE.md

@@ -0,0 +1,33 @@
+# Istio API Style Guide
+
+This page defines the design guidelines for Istio APIs. They apply to
+all proto files in the Istio project. Developers who create their APIs
+using Istio may find these guidelines useful as well.
+
+Since Istio APIs are based on _proto3_ and _gRPC_, we will use
+Google's [API Design Guide](https://cloud.google.com/apis/design) as
+the baseline. Because Envoy APIs also uses the same baseline, the
+commonality across Envoy, Istio, proto3 and gRPC will greatly help
+developer experience in the long term.
+
+In addition to Google's guide, the following conventions should be
+followed for Istio APIs.
+
+## Versioning
+
+When defining Kubernetes Custom Resource Definition (CRD) using
+proto3, the proto versioning should match the Kubernetes versioning,
+see the following example.
+
+The Kubernetes CRD:
+
+```yaml
+apiVersion: config.istio.io/v1alpha1
+kind: Authorization
+```
+
+The proto message definition:
+```proto
+package istio.config.v1alpha1;
+message Authorization {...}
+```