Explain entrypoint

README.md

@@ -64,9 +64,23 @@ After that, we configure the external repository "Zinnwerk" which provides us wi
 
 As a last step, we configure the entry point. The entry point is a script that is started inside the container whenever someone creates runs a container.
 
+### `entrypoint.sh`
+
+This script is run inside the Docker container. It first creates an `xlerc` file containing instructions to create a parser for the specified grammar and parse the sentence. This file is automatically loaded by XLE on startup.
+
+:books: [syntax.xle – syntaktischer zucker (I)][narnold-xle]
+
+It then starts the headless X server, waits to ensure it has properly started and then executes the `xle` command. `xle` will load `xlerc`, create a parser, parse the sentence and display windows with the parse results.
+
+As last step, we take a screenshot of the displayed windows, which is the output of the container.
+
+:books: [How do I take a screen shot of my Xvfb buffer?][screenshot]
+
 [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
+[narnold-xle]:#
+[screenshot]:https://unix.stackexchange.com/questions/365268/how-do-i-take-a-screen-shot-of-my-xvfb-buffer