Currently the snippet macro requires an id attribute, and snippets are only included if they are enclosed with the START and END demarcators. However, neither id nor demarcators are necessary if you want to include an entire file. I'd suggest to allow the following:
which will include the whole file.txt.
Note also that the documentation at http://maven.apache.org/doxia/macros/index.html seems misleading, as it says that the demarcators are only needed if you want to include part of a file.