Document changes in output

README.md

@@ -33,6 +33,8 @@ sudo docker run \
 
 This will take a few seconds. The parses will then be output to the command line using XLE's own XML export format. If you want to store the output in a file, append `> name_of_your_file.xml` at the end of the command.
 
+If no parses are generated, the container will exit with exit code `42` and write the XLE shell output to stderr.
+
 <details>
 <summary>Advanced information</summary>
 
@@ -89,7 +91,9 @@ It then executes `xle` using the `xvfb-run` wrapper (which ensures `xle` can acc
 
 :books: [Ubuntu Manpage: xvfb-run - run specified X client or command in a virtual X server environment][xvfb-run]
 
-As a final step, `cat` is used to output the export file to the standard output.
+As a final step, `cat` is used to output the export file to the standard output or to output the XLE log to standard error.
+
+:books: [Answer to "echo that outputs to stderr" by Brandon Rhodes on Stack Overflow][stderr-so]
 
 [xle]:https://ling.sprachwiss.uni-konstanz.de/pages/xle/
 [xvfb]:https://blog.kagesenshi.org/2007/06/running-x-applications-headless-using.html
@@ -99,3 +103,4 @@ As a final step, `cat` is used to output the export file to the standard output.
 [3rd-repo-lu]:https://www.linuxuprising.com/2021/01/apt-key-is-deprecated-how-to-add.html
 [narnold-xle]:https://www.cl.uni-heidelberg.de/~narnold/xle/posts/2021-05-25-syntax-xle-mad.html
 [xvfb-run]:https://manpages.ubuntu.com/manpages/trusty/man1/xvfb-run.1.html
+[stderr-so]:https://stackoverflow.com/a/11422223/