Update documentation

Give instructions for proper document setup for conversion (#2)

docs/help/import_menu.md

@@ -85,6 +85,11 @@ It is recommended not to modify such markups in yWriter to avoid unwanted nestin
 
 ## HowTo
 
+## General note
+
+Pleawe make sure that the headings are not hard-formatted in your original document. With LibreOffice, select the headings (you can use the Find dialog with "Format" activated), and reset the formatting with `Ctrl`-`M`.
+Then you can remove stray language tags: Select the whole document with `Ctrl`-`A`, then set the language of the selection to your default language.
+
 ## How to set up a work in progress for export
 
 A work in progress has no third level heading.
@@ -107,4 +112,4 @@ An outline has at least one third level heading.
 
 ---
 
-[<< Previous](export_menu) -- [Next >>](descriptions_menu)
+[<< Previous](export_menu) -- [Next >>](descriptions_menu)