Update README.md

Added instructions when seeing "Unable to locate nuget.exe" error during first build
Added IFdbTuple.Last<T>, and made it implements IReadOnlyList<object>

- Last<T>() is the same as Get<T>(-1) but should be "faster"
removed debug trace
Fixed regression in async LINQ on ForEachAsync

- passing a Func<T, Task> to ForEachAsync instead of Func<T, CancellationToken, Task> was using the override of ForEach that takes an Action<T> !
Added an optimized FdbTuple.UnpackLast<T>() that only parse the last segment of a tuple

- Use frequently by code that only need the last item of a tuple returned from a get_range or get_key query