initial, simple SDK documentation (swiftlang#9004)

Provides some basic detail for Swift cross-compilation SDKs, referencing
the installation links on swift.org, the evolution proposal, and the SDK
generator code.

resolves swiftlang#8864, to finalize the documentation of the CLI commands

Author: heckj

Sources/PackageManagerDocs/Documentation.docc/SwiftSDKCommands.md

@@ -9,9 +9,15 @@ Perform operations on Swift SDKs.
 
 ## Overview
 
-Overview of package manager commands here...
+By default, Swift Package Manager compiles code for the host platform on which you run it.
+Swift 6.1 introduced SDKs (through
+[SE-0387](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0387-cross-compilation-destinations.md))
+to support cross-compilation.
 
-<!-- reference content for the CLI commands `swift package ...` -->
+SDKs are tightly coupled with the toolchain used to create them.
+Supported SDKs are distributed by the Swift project with links on the [installation page](https://www.swift.org/install/) for macOS and Linux, and included in the distribution for Windows.
+
+Additionally, the Swift project provides the tooling repository [swift-sdk-generator](https://github.com/swiftlang/swift-sdk-generator) that you can use to create a custom SDK for your preferred platform.
 
 ## Topics