Document minimum hardware/ OS requirements

[BrianGilbert]

@rfay are the minimum requirements known?

This will save people that are going to have issues from attempting to install

[rfay]

Any system that can run docker is good. . Here are the docs: https://ddev.readthedocs.io/en/stable/#system-requirements

On macOS that means from Sierra up works; on Windows as you know Win10 is preferable but you can run Docker Toolbox on Win 7+. Most Linux versions any time in recent ages works.