add git

[hulaizh]

No description provided.

[hannesdatta]

Hi Hulai,
our branches have diverted in the mean time. Kindly screen the current version of GitInstall.md, and see which parts make sense to integrate. Note that we have a new page under workflow/versioning.md and workflows/contributing.md which now are the tutorial. so any changes would have to be made there. kindly see which material nicely adds to what we have now, and re-issue your PR. Thanks.