Update README.md

Author: wkh237

README.md

@@ -86,7 +86,7 @@ HTTP request method, can be one of `get`, `post`, `delete`, and `put`, case-inse
 #### url:`string` Required
 HTTP request destination url.
 #### headers:`object` (Optional)
-Headers of HTTP request, value of headers should be `stringified`.
+Headers of HTTP request, value of headers should be `stringified`, if you're uploading binary files, content-type should be 'application/octet-stream' (multipart/form-data will be available in next release).
 #### body:`string` (Optional)
 Body of the HTTP request, body MUST be a BASE64 string, this string will be converted into byte array in native code.