Create Best Practices Docs

[atmoos]

Some concepts are non-intuitive. Add How-To or best practices for following topic:

• Distinction Quantity vs. Unit
• Create new units
  • linear units (eg. metre -> feet)
  • with focus on combined quantities (velocity)
    • better not define FooLengthPerBarTime velocity unit
    • but define both a FooLength length unit and a BarTime time unit, then use "Per" method to combine them
• Using variables to capture units
• Point out the existence of the Units nuget at the point where I describe creation of new units.