message: docs: add note on README about the READFUNCTION option
diff: - [MultiPart Upload / HttpPost libcurl Option (Content-Type: multipart/form-data)](#multipart-upload--httppost-libcurl-option-content-type-multipartform-data)
  - [Binary Data](#binary-data)
  - [API](#api)
  +- [Special Notes](#special-notes)
  + - [`READFUNCTION` option](#readfunction-option)
  - [Common Issues](#common-issues)
  - [Benchmarks](#benchmarks)
  - [Supported Libcurl Versions](#supported-libcurl-versions)
  @@ -205,6 +207,14 @@ Almost all [CURL options](https://curl.haxx.se/libcurl/c/curl_easy_setopt.html)
  For more usage examples check the [examples folder](./examples).
  +## Special Notes
  +
  +### `READFUNCTION` option
  +
  +The buffer passed as first parameter to the callback set with the [`READFUNCTION`](https://curl.haxx.se/libcurl/c/CURLOPT_READFUNCTION.html) option is initialized with the size libcurl is using in their upload buffer (which can be set with [`UPLOAD_BUFFERSIZE`](https://curl.haxx.se/libcurl/c/CURLOPT_UPLOAD_BUFFERSIZE.html)), this is initialized using `node::Buffer::Data(buf);` which is basically the same than `Buffer#allocUnsafe` and therefore, it has all the implications as to its correct usage: https://nodejs.org/pt-br/docs/guides/buffer-constructor-deprecation/#regarding-buffer-allocunsafe
  +
  +So, be careful, make sure to return **exactly** the amount of data you have written to the buffer on this callback. Only that specific amount is going to be copied and handed over to libcurl.
  +
  ## Common Issues
  See [COMMON_ISSUES.md](./COMMON_ISSUES.md)
  