Add a comparison of `std` vs `no_std` approaches

[jessebraham]

It seems to be unclear to a number of newer users why you would select one approach over the other. It would be good to have a section dedicated to describing both approaches and comparing their advantages and disadvantages.

[jessebraham]

This should be mostly covered by #14. This section will need updating in the future but there's not much else to add currently.