Sure. In your mainpage.dox (or whatever you call it), place a link to your README.txt as a separate doxygen page.
\mainpage FooBar
...
\section other_docs Additional Documentation`
...
Key features that distinguish the current release, build instructions,
and a history of Project FooBar are described in the
\ref README "Foobar README file".
and then later on, generate that page by making your README.txt a plain text page file.
\page README README
\verbinclude README.txt