Update README.md

Author: wkh237

README.md

@@ -19,15 +19,15 @@ A module provides upload, download, and files access API. Supports file stream r
 * [API References](https://github.com/wkh237/react-native-fetch-blob/wiki/Fetch-API)
 * [Development](#user-content-development)
 
-**Rationale**
+## Rationale
 
 React Native does not support `Blob` object at this moment, which means if you're going to send/receive binary data via `fetch` API, that might not work as you expect. See [facebook/react-native#854](https://github.com/facebook/react-native/issues/854).
 
 For some use cases, you might get into trouble. For example, displaying an image that requires a specific field in headers (ex. "Authorization : Bearer ...") or body, so you can't just pass the image uri to `Image` component because that will probably returns a 401 response. Or you're going to upload binary data which generated from JS, the server will get an empry body due to [this issue](https://github.com/facebook/react-native/issues/854). With help of APIs provided by this module, you can send HTTP request with any headers, and decide how to handle the response/reqeust data without worry about if it is not supported by `fetch` API. The response data can be just simply converted into BASE64 string, or stored to a file directly so that you can read it by using file access APIs such as readFile, readStream.
 
 This module was designed to be a substitution of `Blob`, there's a set of APIs including basic file system CRUD method, and file stream reader/writer. Also it has a special `fetch` implementation that supports binary request/response body.
 
-**Backward Compatible**
+## Backward Compatible
 
 All updates are `backward-compatible` generally you don't have to change existing code unless you're going to use new APIs. But we recommend pre `0.5.0` users consider upgrade the package to latest version, since we have introduced new APIs can either `upload` or `download` files simply using a file path. It's much more memory efficent in some use case. We've also introduced `fs` APIs for access files, and `file stream` API that helps you read/write files (especially for **large ones**), see [Examples](#user-content-recipes) bellow. This module implements native methods, supports both Android (uses awesome native library  [AsyncHttpClient](https://github.com/AsyncHttpClient/async-http-client])) and IOS.