Add documentation to the topydo repository.

doc/README.md

-For the full documentation, please refer to https://www.topydo.org .
-A static / downloadable copy of the documentation can be found at https://www.topydo.org/static.html .
+This folder contains the
+[documentation](https://raw.githubusercontent.com/bram85/topydo/doc/Documentation.html)
+as hosted on https://www.topydo.org .
+
+A static / downloadable copy of the documentation can be found at
+https://www.topydo.org/static.html .