Update README.md

Author: Axel Rindle

README.md

@@ -3,7 +3,11 @@ Simple **version checker** working with **GitHub releases** and the **GitHub API
 
 ## Install
 ```bash
-npm install --save github-version-checker
+$ npm install github-version-checker
+```
+or
+```bash
+$ yarn install github-version-checker
 ```
 
 ## Usage
@@ -18,7 +22,8 @@ const options = {
   currentVersion: pkg.version, // your app's current version
   includePreReleases: false // if you want to check pre-releases to
 };
-versionCheck(options, function (update) { // callback function
+versionCheck(options, function (update, error) { // callback function
+  if (error) throw error;
   if (update) { // print some update info if an update is available
     console.log('An update is available!');
     console.log('New version: ' + update.tag_name);
@@ -35,16 +40,29 @@ Check the [examples folder](https://github.com/axelrindle/github-version-checker
 ### Note on semantic versioning
 Always try to follow a clear [semver format](http://semver.org/). This helps to avoid parsing mistakes and getting a clear result.
 
-## Options
+## API
+### `function versionCheck(options, [callback])`
+Performs an update check with the given options. The `callback` is optional, can be left out to return a `Promise`.
+
+#### The `options` object
 Option | Default value | Description
 --- | --- | ---
 repo | **None. Required option** | Defines from which GitHub repository to load releases from.
 currentVersion | **None. Required option** | Your app's current version. Needed to check if a newer release is found.
 includePreReleases | false | Whether or not to include pre-releases. **Note:** This will fetch **all** releases, which might result in high traffic, depending on how much releases your app has on GitHub.
 
-## The `update` object
-The object you receive in the callback function, follows the format specified here:
-https://developer.github.com/v3/repos/releases/#get-a-single-release
+#### The `callback` function (optional)
+Should be of the following form:
+```javascript
+function(update, error) {
+  // ...your code
+}
+```
+* `update`:
+  * An object in the format specified [here](https://developer.github.com/v3/repos/releases/#get-a-single-release). `null` if no update was found.
+* `error`:
+  * If an error occurs, this holds the error message. `null` if no error occurs.
+
 
 ## Todo