There's a trick you can use to do this without using any other sort of dashboard widget:
This MacTipper article from 2008 shows how if you open a text file in Safari, you can then send that file to be opened in Dashboard.
... anything you can open in Safari, you can have on the dashboard. This includes text (TXT) files. If you combine this with other fun things you can do with text files, such as your basic todo list, you can put some pretty good information on the dashboard. As with images, you just need to open the text file in Safari and add it to the Dashboard.
Apple have removed that ability in later version of Safari, however using a simple HTML file allows you to bypass that restriction as in the example below:
<iframe src="todo.txt" width="600" height="400">
For other people who come to this thread looking to get TODO lists in the Dashboard, then iSlayer's Organized is a Dashboard widget that provides TODO functionality in the Dashboard.
To do’s are synced with OS X’s built-in database, so Organized will
always reflect what’s shown in Mail and other apps.