Add git pull instructions and change github url to ardupilot

[Ardustorm]

I'm building BBBMINI on the beaglebone and added the git pull directions in the software documentation. I also updated the git urls to 'ardupilot' from 'diydrones' since that is where it gets redirected anyways.

[Ardustorm]

I also ran into issues installing future using pip, though I'm not sure if that is a problem specific to me. To fix it I ran these commands:

sudo pip install setuptools --upgrade sudo pip install --upgrade distribute

I can add them to the instructions if that seems useful.

[mirkix]

@Ardustorm Thanks!