update readme with note about the automatic version lowering, issue #25…

… [skip ci]

README.md

@@ -93,3 +93,11 @@ Usage:
     $dir = PhantomInstaller\PhantomBinary::getDir();
 
 This feature is similar to `location.js` of the [phantomjs module](https://github.com/Medium/phantomjs/blob/master/install.js#L93) for Node.
+
+## Automatic version lowering on 404
+
+In case downloading an archive fails with HttpStatusCode 404 (resource not found),
+the downloader will automatically lower the version to the next available version
+and retry. The number of retries is determined by the number of hardcoded PhantomJS
+versions in `getPhantomJSVersions()`. This feature was added, because of the problems
+with v2.0.0 not being available for all platforms (see issue #25).