Update STYLE-GUIDE.md (#373)

Add a rule for repeated fields.

Author: wora

STYLE-GUIDE.md

@@ -1,8 +1,8 @@
 # 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.
+all proto files in the Istio project. Developers who create their own
+APIs using Istio should 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
@@ -16,8 +16,8 @@ followed for Istio APIs.
 ## Naming
 
 Having consistent naming improves understanding of the APIs and reduce
-churns on the API design. Istio APIs should follow the following
-naming convention:
+churns on the API design. Istio APIs should follow these naming
+conventions:
 
 * Package names must use `lowercase` without any `_`. Singular words
   are recommended to avoid mixture of plural and singular words in
@@ -33,7 +33,10 @@ naming convention:
 * Field names must use `lowercase_with_underscore`. For example,
   `display_name`.
 
-* Avoid using postpositive adjectives. For example,
+* Repeated fields must use proper plural names. For example,
+  `rules`.
+
+* Avoid using postpositive adjectives in names. For example,
   `collected_items` is recommended over `items_collected`.
 
 ## Versioning