Update README.md

Altered documentation on custom ex libris files.

README.md

@@ -99,7 +99,6 @@ The available options are as follows. Refer to the API Documentation for details
 - downloadlimit (int)
 
 #### optional:
-- exlibrisfile (string), this should contain the location of a png-file according to the specifications in the API Documentation.
 - exlibrisfont (string), this should contain either 'sans', 'serif' or 'script'
 - exlibris (bool), default: false
 - chapterfooter (bool), default: false
@@ -110,6 +109,12 @@ The available options are as follows. Refer to the API Documentation for details
 - epub (bool), default: true
 - kf8mobi (bool), default: false
 
+#### custom ex libris
+It is also possible to set a custom ex libris file according to the specifications in the API Documentation.
+```php
+$BooXtream->setExlibrisFile('filename');
+```
+
 ### Response
 
 The BooXtreamClient returns an object of the type of [Guzzle/Http/Message/Response](http://api.guzzlephp.org/class-Guzzle.Http.Message.Response.html)