If you use serverside scripting such as php then you can.
Another way to include txt files is to call it using a javascript - like:
Put this in the body of your html:
<script language="javascript" src="text.txt"></script>
Then create a new text file in the same folder as your html file called text.txt and put this in it:
document.write('Your text file');
Open the html file in your browser and it should display the text "Your text file" (assuming you have javascript turned on).
Make sure you don't delete the document.write(''); as it will make the script not work.