Add example snippet for file upload (#36)

* Add example snippet for file upload

* Update example to match others

Author: cemremengu

README.md

@@ -43,6 +43,28 @@ try {
 }
 ```
 
+File uploads (multipart/form-data) can be achieved by using [form-data](https://github.com/form-data/form-data) package as shown below:
+
+```js
+const FormData = require('form-data')
+const fs = require('fs')
+ 
+try {
+  const form = new FormData()
+  form.append('myfile', fs.createReadStream(`./path/to/file`))
+
+  const res = await inject(dispatch, { 
+    method: 'post', 
+    url: '/upload', 
+    payload: form, 
+    headers: form.getHeaders() 
+  })
+  console.log(res.payload)
+} catch (err) {
+  console.log(err)
+}
+```
+
 ## API
 
 #### `inject(dispatchFunc, options, callback)`