Added git primer for developers.
Added git primer for developers.

This is an introduction to git and GitHub for developers unfamiliar
with the two.

stefanomarty/openxpki,mrscotty/openxpki,stefanomarty/openxpki,aleibl/openxpki,stefanomarty/openxpki,aleibl/openxpki,oliwel/openxpki,openxpki/openxpki,aleibl/openxpki,aleibl/openxpki,mrscotty/openxpki,mrscotty/openxpki,stefanomarty/openxpki,oliwel/openxpki,openxpki/openxpki,stefanomarty/openxpki,stefanomarty/openxpki,aleibl/openxpki,oliwel/openxpki,oliwel/openxpki,aleibl/openxpki,oliwel/openxpki,oliwel/openxpki,openxpki/openxpki,openxpki/openxpki