Descripton of file name forming

Author: petrvanekrobe

gdtf-spec.md

@@ -178,6 +178,10 @@ The ZIP archive name is specified as follows:
 
 Example: `generic@led@comment`
 
+It is recommended to limit filenames to the POSIX "Fully Portable Filenames"
+character set: [A-Z], [a-z], [0-9], the symbols '_' (U+005F), '-' (U+002D) and
+a maximum of one '.' (U+002E).
+
 UTF-8 has to be used to encode the XML file. Each XML file internally
 consists of XML nodes. Each XML node could have XML attributes and XML
 children. Each XML attribute has a value. If a XML attribute is not