Add another example code style rule

docs/README.md

@@ -26,6 +26,7 @@ In the post body, write the HTML and JavaScript constituting the example.
 * Use **4 space indentation**. Exception: do not add an initial level of indentation to code within `<script>` tags (it should start flush left).
 * Do **not** include an access token in the example code. The access token will be inserted automatically by the template, using the current logged in user's default public token, or a placeholder `<insert token here>` string if the user is not logged in.
 * Do **not** use custom styles from your personal account. Use only the default `mapbox` account styles.
+* When embedding literal JSON (GeoJSON or GL style snippets) into script code, double-quote property names and string values. Elsewhere, use single-quoted strings.
 
 ## Generating Documentation