Mark TaskBuilder as Obsolete and point to TaskBuilder.fs #368

The task builder in FSharpx.Extras is obsolete and should be marked as Obsolete with a comment that points users in the right direction.

Previous text:

Description

How are you supposed to use TaskBuilder with Task, i.e. not Task<unit> ?

Repro steps

Expected behavior

        member this.Batch cs= task {
            use fs = File.Open(fileName, FileMode.Append, FileAccess.Write, FileShare.Read)
            use w = new StreamWriter(fs)
            w.WriteLine(serialize(cs |> List.toArray))
            do! fs.FlushAsync()
        }

Actual behavior

I'd expect it task computation builder to work with Task but it seems I need to do:

        member this.Batch cs= task {
            use fs = File.Open(fileName, FileMode.Append, FileAccess.Write, FileShare.Read)
            use w = new StreamWriter(fs)
            w.WriteLine(serialize(cs |> List.toArray))
            do! fs.FlushAsync().ContinueWith<unit>( fun _ -> () )
        }