This can (at least now) be done with command line options to the dependency:tree plugin.
Try:
mvn dependency:tree -Doutput=/path/to/file
Reference: Maven Dependency Plugin Page
You only asked about “readable” format, but you can also pass the -DoutputType parameter with various options. Also note that the version I have installed, I get the following warning:
[WARNING] The parameter output is deprecated. Use outputFile instead.
So, consider trying it with -DoutputFile=/path/to/file
Also, I was unable to get the -DoutputType paramater to give me anything other than the default text, but didn’t have a chance to play around with it. YMMV.