README: added env var

Author: kelsk

run/markdown-preview/README.md

@@ -11,6 +11,7 @@ Required variables for this sample include:
 
 * `EDITOR_UPSTREAM_RENDER_URL`: The URL of the restricted Cloud Run service that
   renders Markdown to HTML.
+* `EDITOR_UPSTREAM_UNAUTHENTICATED`: A boolean that indicates whether the service is unauthenticated.
 
 ## Dependencies
 
@@ -19,4 +20,4 @@ Required variables for this sample include:
 * **got**: Node.js library for HTTP requests.
 * **handlebars** JavaScript template engine.
 * **markdown-it**: JavaScript library for parsing and rendering Markdown text.
-* **xss**: Node.js HTML sanitizer.
+* **xss**: Node.js HTML sanitizer.