Document ln caveats

Currently, ln builds on mkfifo to create a fs fifo. In dat-shell, we can currently 

```
ln a-file-inside-dat.txt ~/filesystem-file.txt
```

And from another terminal:
```
tail ~/filesystem-file.txt
cp ~/filesystem-file.txt somewhere.txt
```

But unfortunately, neither mplayer nor vlc will play a video file linked in a such a way.