add how-to on how to convert a normal raspbian to cattlepi

[t3spe]

this conversion can be done on the fly. document it and/or maybe provide a script that can be downloaded and ran

something along the lines of:

cd /tmp
curl -L https://api.cattlepi.com/images/global/raspbian-lite/2018-06-29/bootstrap/cattlepi.zip?apiKey=deadbeef > cattlepi.zip
unzip cattlepi.zip
mkdir -p /tmp/boot
sudo mount  -o loop,offset=1048576 /tmp/cattlepi.img /tmp/boot
sudo mkdir /boot/cattlepi
sudo vim /boot/cattlepi/apikey # put your api key in here
sudo cp -R /tmp/boot/* /boot/
sudo /sbin/shutdown -r now

[t3spe]

this is a little trickier that I originally though. The limiting factor is that the boot partition for stock raspbian is not large enough to hold images that cattlepi would leverage. What this means is that we would need a way to resize the partition (maybe via sdlayout) before we can attempt this.

[t3spe]

due to increase in scope complexity I've moved this to 1.4. we may decide not to do this at all.