update Developing LHCb Software to use git

[pikacic]

All major LHCb projects migrated to Git this year, so it's time to merge https://github.com/lhcb/second-analysis-steps/blob/master/02-lb-git.md in 15-lhcb-dev.md (and remove the git lesson from second-analysis-steps).

[pikacic]

I could prepare a pr some times next week.

[alexpearce]

I wonder if it's worth teaching this stuff during the git lessons. Usually we don't teach anything LHCb specific in the first couple of days, but maybe it makes sense there.