Don't advertise the master branch when it's not building

Since people are coming on IRC wondering why they can't build Nimrod, many more must be put off without even asking for help. Since the master branch is currently not building (c-sources are for devel from what I've heard?) I would suggest advertising the working branch.

Oh, and also there is some more information on https://github.com/araq/nimrod than in download.txt. Maybe the Source part of download.txt can be removed by a link to the github page?