Previously, README.Debian use to be installed to /u/s/d/i2p. A while ago I
split I2P into smaller packages and symlinked /usr/share/doc/libjbigi-jni and
/usr/share/doc/i2p to /usr/share/doc/i2p-router. I didn't noticed that
README.Debian wasn't being included after the split, so now I'm explicitly
adding it.