Document setting up extensionless feed paths

Resolves #237.

Author: sonicdoe

README.md

@@ -45,6 +45,8 @@ feed:
 
 To note, you shouldn't have to do this unless you already have a feed you're using, and you can't or wish not to redirect existing subscribers.
 
+:warning: If your feed path does not end with `.xml`, you'll need to configure your web server to serve the proper content type. Alternatively, you can specify e.g. `feed/index.xml` to serve your feed from `/feed/`.
+
 ### Optional front matter
 
 The plugin will use the following post metadata, automatically generated by Jekyll, which you can override via a post's YAML front matter: