Clarify what values responseType can have in Node (#2121)

It seems that `responseType: 'blob'` doesn't actually work in Node (when I tried using it, response.data was a string, not a Blob, since Node doesn't have Blobs), so this clarifies that this option should only be used in the browser

README.md

@@ -304,7 +304,8 @@ These are the available config options for making requests. Only the `url` is re
   },
 
   // `responseType` indicates the type of data that the server will respond with
-  // options are 'arraybuffer', 'blob', 'document', 'json', 'text', 'stream'
+  // options are: 'arraybuffer', 'document', 'json', 'text', 'stream'
+  //   browser only: 'blob'
   responseType: 'json', // default
 
   // `responseEncoding` indicates encoding to use for decoding responses