add UnicodeDecodeError to troubleshooting (via #93)

docs/docs/troubleshooting.rst

@@ -12,6 +12,21 @@ Install Isso in a virtual environment as described in
 :ref:`install-interludium`. Alternatively, you can use `pip install --user`
 to install Isso into the user's home.
 
+UnicodeDecodeError: 'ascii' codec can't decode byte 0xff
+--------------------------------------------------------
+
+Likely an issue with your environment, check you set your preferred file
+encoding either in :envvar:`LANG`, :envvar:`LANGUAGE`, :envvar:`LC_ALL` or
+:envvar:`LC_CTYPE`:
+
+.. code-block:: text
+
+    $ env LANG=C.UTF-8 isso [-h] [--version] ...
+
+If none of the mentioned variables is set, the interaction with Isso will
+likely fail (unable to print non-ascii characters to stdout/err, unable to
+parse configuration file with non-ascii characters and to forth).
+
 The web console shows 404 Not Found responses
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