Descripton of file name forming

Author: petrvanekrobe

gdtf-spec.md

@@ -173,11 +173,18 @@ all firmware revisions of the device.
 
 ```
 
-The ZIP archive name is specified as follows:
+The ZIP archive name is recommended to be specified as follows:
+
 `<ManufacturerName>@<FixtureTypeName>@<OptionalComment>`
 
 Example: `generic@led@comment`
 
+It is recommended to limit the filename 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), if needed, use '_' (U+005F) or '-' (U+002D) to
+replace unsupported characters.
+
+
 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