Add the NFS file lock gotcha to the troubleshooting guide

If `gangadir` is in a file system that doesn't allow file locks, it won't work. The simple workaround is to edit `~/.gangarc` and put `gangadir` in a different place, so we need a note on that (you can change `$HOME` too, but that's a bit sledgehammery-nut-crackery).