MANUAL: note that custom readers/writers will...

be sought in the readers/writers subdirectory of
the user data directory if not found relative to
the working directory.

Documents #8112.

MANUAL.txt

@@ -6967,6 +6967,10 @@ Lua script in place of the input or output format. For example:
     pandoc -t data/sample.lua
     pandoc -f my_custom_markup_language.lua -t latex -s
 
+If the script is not found relative to the working directory,
+it will be sought in the `readers` or `writers` subdirectory
+of the user data directory (see `--data-dir`).
+
 A custom reader is a Lua script that defines one function,
 Reader, which takes a string as input and returns a Pandoc
 AST.  See the [Lua filters documentation] for documentation