Adding comment about header names (#778)

Header names are lower cased by axios Which make sense since the header names are case insensitive.

README.md

@@ -351,6 +351,7 @@ The response for a request contains the following information.
   statusText: 'OK',
 
   // `headers` the headers that the server responded with
+  // All header names are lower cased
   headers: {},
 
   // `config` is the config that was provided to `axios` for the request