Update README.md

README.md

@@ -38,12 +38,14 @@ done
 
 ## Configuration
 
-The following environment variables are supported:
+The following optional environment variables are supported:
 
 - `IGNORED`: A list of [glob expressions](http://man7.org/linux/man-pages/man7/glob.7.html)
   separated by newlines. Files matching these expressions will not count when
   calculating the change size of the pull request. Lines starting with `#` are
   ignored and files matching lines starting with `!` are always included.
+- `HTTPS_PROXY`: A proxy URL to pass to [https-proxy-agent](https://www.npmjs.com/package/https-proxy-agent)
+  which will be used to proxy requests to the GitHub API.
 
 You can configure the environment variables in the workflow file like this: