Add Subunit support documentation

[jflopezfernandez]

I actually didn't know what subunit was, so I was pretty confused by the option to enable it during configuration while researching issue #302. I have verified that I can successfully clone the subunit repo, configure, build, and install the subunit library, and then run configure with the --enable-subunit option.

I think it would be a good idea to expand on what that functionality is. Nothing absolutely groundbreaking or epically expansive, just something to clue in others who might not be familiar with that particular library.