Update README.md with details about postFile

README.md

@@ -100,12 +100,32 @@ For the sake of clarity, we provide the following two-steps call syntax:
 
 - `getDocumentById(id).getSimilar(cb)` will call `GET /documents/{id}/similar`
 - `getDocumentById(id).getRaw(cb)` will call `GET /documents/{id}/raw`
-- `getDocumentById(id).postFile(cb)` will call `POST /documents/{id}/file`
+- `getDocumentById(id).postFile(config, cb)` will call `POST /documents/{id}/file`
 
 Note that the first function **does not take any callback**. It is simply responsible for building the first part of the request, which is then carried out when calling the sub-function.
 
 A full description of the mapping functions is available in [`api-descriptors.json`](config/json/api-descriptors.json).
 
+### Posting a file associated to a document
+
+The function `getDocumentById(id).postFile(config, cb)` expects a `config` hash containing at least a `file` key from which to obtain the file. It can be a string (path to the file) or a `ReadStream`. It can also contains `contentType` (MIME type) and `filename` keys.
+
+`config` can also be passed as a function. In this case, it is invoked with a callback, which must be called with `(err, config)`.
+
+**Example usage:**
+```js
+var deliverConfig = function(cb) {
+   cb({
+      file: fs.createReadStream('path/to/file.png'),
+      filename: 'doge.png'
+   });
+};
+
+getDocumentById(id).postFile(deliverConfig, function(err) {
+  // Handle error if any
+});
+```
+
 ## Utility functions
 `anyfetch.js` provides higher level utility functions. They cover classic use-cases that would otherwise require several API calls. When possible, calls are grouped in a single batch call.