Draft: Support UTF-8

To quote from https://gitlab.cl.uni-heidelberg.de/Tutoren/syntax/flexi/-/issues/31

That problem is known. The problem is that XLE does, by default, not use UTF-8 (neither when reading the LFG file nor when reading the sentence). This means it interprets the "ü" as something different both in the grammar and in the sentence (namely the "ü"), but at least it parses.

You can add a line to the grammar to tell XLE it should interpret it as UTF-8, but then you'd also have to tell XLE to interpret the input as UTF-8. That can probably be done, I'm working on it in xle-docker, it just isn't as easy as I thought, because the encoding does not apply to the xlerc file in which I save the sentence to be parsed.

See also: https://ling.sprachwiss.uni-konstanz.de/pages/xle/doc/xle.html#SEC23

Maybe helpful command: set-character-encoding * utf-8

README.md

@@ -59,6 +59,18 @@ sudo docker container run xle $(cat data.json | base64 -w0)
 The idea to implement it that way came from [Christian Heusel](https://christian.heusel.eu/), thank you very much!
 </details>
 
+## :abcd: A word about encoding
+
+By default, this container lets XLE use its default encoding, whatever that is (not UTF-8). If you want to use UTF-8, you have to add the following line to your grammar file in the `CONFIG` section
+
+```
+CHARACTERENCODING utf-8.
+```
+
+**and** execute `set-character-encoding stdio utf-8` every time after starting XLE.
+
+`xle-docker` will automatically use `utf-8` as character encoding for standard input if it detects the character encoding configuration line (more precisely, a line matching `^\s*CHARACTERENCODING\s+utf-8\s*\..*$` anywhere in the file).
+
 ## :page_with_curl: About
 
 Here, a more detailed explanation of the relevant files is given. The used sources are marked with :books:.