SwiftPM manifest file

Hello,

is it possible to add a package description file for Swift Package Manager?

I've done something similar:

```
// swift-tools-version:5.1

import PackageDescription

let package = Package(
name: "TrueTime",
products: [
.library(
name: "TrueTime-iOS",
targets: ["TrueTime-iOS"]
),
.library(
name: "TrueTime-Mac",
targets: ["TrueTime-Mac"]
),
.library(
name: "TrueTime-tvOS",
targets: ["TrueTime-tvOS"]
)
],
dependencies: [
// .package(url: /* package url */, from: "1.0.0"),
],
targets: [
.target(
name: "TrueTime-iOS",
dependencies: [],
path: "Sources"
),
.target(
name: "TrueTime-Mac",
dependencies: [],
path: "Sources"
),
.target(
name: "TrueTime-tvOS",
dependencies: [],
path: "Sources"
)
]
)
```

but it's only the first step since there are also the C files to put into an `include` directory where Xcode can find them.

Is there any plan to support this?