Add further sources

README.md

@@ -53,7 +53,8 @@ The Dockerfile specifies the steps with which the image is built. As a first ste
 * `imagemagick`: To take screenshots from the XLE parse windows
 * `ca-certificates`: To verify the HTTPS connection to the external XLE repository
 
-:books: [Running X applications headless using Xvfb][xvfb]
+:books: [Running X applications headless using Xvfb][xvfb] <br />
+:books: [Fake X server for testing?][xvfb-so]
 
 After that, we configure the external repository "Zinnwerk" which provides us with a copy of the XLE application.
 
@@ -65,6 +66,7 @@ As a last step, we configure the entry point. The entry point is a script that i
 
 [xle]:https://ling.sprachwiss.uni-konstanz.de/pages/xle/
 [xvfb]:https://blog.kagesenshi.org/2007/06/running-x-applications-headless-using.html
+[xvfb-so]:https://stackoverflow.com/questions/14321636/fake-x-server-for-testing
 [3rd-repo-debwiki]:https://wiki.debian.org/DebianRepository/UseThirdParty#Instructions_to_connect_to_a_third-party_repository
 [3rd-repo-se]:https://unix.stackexchange.com/questions/332672/how-to-add-a-third-party-repo-and-key-in-debian
 [3rd-repo-lu]:https://www.linuxuprising.com/2021/01/apt-key-is-deprecated-how-to-add.html