README Api Usage section out of date

[Wes]

At first glance of the README I went straight to API usage and saw the Platform enums shown as

public enum Platform
{
  Android,
  iOS,
  WindowsPhone,
  Windows
}

That isn't correct anymore. Also the Version string for WinRT works now, right? The README says it always returns "8.1", but looking at the code it seems to be properly parsing the version.

So this bug is to track updating the "Api Usage" section of README

[James Montemagno]

I am slowly migrating all my docs over to a new format right now :)