x2t: add README.txt

cloudooo/handler/x2t/README.txt

+Cloudooo X2T handler
+====================
+
+Table of content :
+
+- Description
+- Handled Mimetypes
+- Contributors
+
+
+Description
+-----------
+
+The `cloudooo.handler.x2t.handler` is a conversion handler that uses Ascensio
+System OnlyOffice x2t converter.
+
+This binary is theoricaly able to convert many document formats including open
+documents and microsoft documents, but the x2t handler was designed to handle
+from "x" format (i.e. DOCX, PPTX, XLSX) to OnlyOffice format only.
+
+Here, the term "y" format (i.e. DOCY, PPTY, XLSY) defines document format used
+by OnlyOffice apps. These "y" formats are just file extensions used by Cloudooo
+to send resulting OnlyOffice converted document correctly.
+
+A "y" file can be :
+
+- a raw file containing base64 of body data without any component (metadata,
+  images, ...) ;
+- or a zip file containing :
+    - "body.txt" - a raw file containing base64 of body data without component ;
+    - "metadata.json" - a json representation of document metadata ;
+    - "media/" - a folder containing document component.
+
+
+Handled Mimetypes
+-----------------
+
+- application/x-asc-text (docy) ;
+- application/x-asc-presentation (ppty) ;
+- application/x-asc-spreadsheet (xlsy).
+
+
+Contributors
+------------
+
+Based on `cloudooo.handler.yformat.handler` from Boris Kocherov original work.
+
+- Boris Kocherov <bk@raskon.ru>
+- Tristan Cavelier <tristan.cavelier@nexedi.com>