[doc] File format: add encoding and root tag specs

doc/source/file_format_specification.rst

@@ -4,6 +4,10 @@ Cadbiom File Format Specification
 Models
 ------
 
+Cadbiom models are written in XML (Extensible Markup Language).
+The current encoding is ASCII, for reasons of ease of maintenance because the language used and the interface remain in Python2 and GTK2.
+This means that **non-ASCII characters are currently not allowed** in these files.
+
 Naming
 ~~~~~~
 Model files should have a .bcx extension.
@@ -12,6 +16,10 @@ MIME Type
 ~~~~~~~~~
 ``application/xml`` or ``text/plain`` (depending on the version, the prologue may be absent).
 
+Body
+~~~~
+The root element is a tag <model>.
+
 Tags for nodes
 ~~~~~~~~~~~~~~
 Type of tags are: