Add bluetooth setup to user manual

[Jan Schubert]

I'd suggest to rename chapter "4.2. HOW TO FIND THE DEVICE NAME" to
something like "4.2. HOW TO FIND THE DEVICE NAME FOR USB ENABLED
DEVICES" in the user manual and add the following chapter. I've not
translated it to German yet as I would someone else read this first and
I would just Englisch terms anyway. So maybe you can have a look to this:

4.3 Setting up bluetooth enabled devices

Using dive computers communicating through bluetooth like the Heinrichs
Weikamp Frog or the Shearwater Predator and Petrel there is a different
procedure to get the devices name to communicate with subsurface. In
general it consists of these Steps:

0. Enable bluetooth on your computer
Please make sure you have bluetooth enabled on your computer running
subsurface. Depending if your system is running initd or systemd this
might be different and might also involve loading modules specific to
your hardware. If you run a common distribution it will most likely be
setup and up and running already. In case your system is running systemd
manually run "sudo systemctl start bluetooth.service" to enable it, in
case of initd run something like "sudo rc.config start bluetoothd" or
"sudo /etc/init.d/buetooth start".

1. Pairing the device
Depending on your distribution and window manager this might be quite
easy by just using an easy to follow system dialog provided. Using
Gnome3 for instance will show a bluetooth icon in the upper right corner
of your desktop where you select "Set up New Device". This should show
you a dialog where you are able to select your device and pair it. If
you have issues with PIN setting try manually setting "0000". Please do
not forget to set your dive computer in bluetooth mode before, if you
use a Shearwater Predator/Petrel just select "Dive Log+" -> "Upload Log"
and wait until you see the "Wait PC" message.

You may also use a manual approach by using these commands:
* "sudo hciconfig" - shows the bluetooth devices available on your
computer (not dive computer), most likely you will see a hci0, if not
try "sudo hcitool -a" to see inactive devices and try to run "sudo
hciconfig hci0 up" to bring them up
* "sudo hcitool scanning"- use this to get a list of bluetooth enabled
client devices, watch out for your dive computer and remember the MAC
address shown there
* "sudo bluez-simple-agent  hci0 10:00:E8:C4:BE:C4" - this will pair
your dive computer with the bluetooth stack of your computer, copy/paste
the MAC address from the output of "hcitool scanning"

2. Bind a rfcomm device
Unforturnately this has to be done manually by running
* "sudo rfcomm bind /dev/rfcomm0 10:00:E8:C4:BE:C4" - bind the dive
computer to a communication device in your computer, in case rfcomm is
not availabe just use rfcomm1 or up, please copy/paste the MAC address
from the output of "hcitool scanning", the MAC shown in here will not
work for you :-).

For downloading dives in subsurface you have then to specify
/dev/rfcomm0 as device name to use.