Update getting_started.rst

1. Added version control stuff
2. Installation information
If you look at the [rich diff](https://github.com/JuliaStats/DataFrames.jl/pull/713/files?short_path=2d47d03#diff-2d47d03493bcd77a5aa90309fc365a50) on github, you can see that it works the way github renders `.rst` files.