Add requirement to use US English

Added a new section "Language", with only one sentence in it.

docs/home/contribute/style-guide.md

@@ -18,6 +18,12 @@ docs, follow the instructions on
 **Note:** Kubernetes documentation uses [GitHub Flavored Markdown](https://github.github.com/gfm/).
 {: .note}
 
+
+## Language
+
+Kubernetes documentation uses US English.
+
+
 ## Documentation formatting standards
 
 ### Use camel case for API objects