Adding new standard library readme

Adding a readme describing the different layers for the standard
libraries, and what libraries belong to which layer.

  - Core: The core runtime libraries that provide basic operations
  - Overlay: Maps the underlying system into Swift
  - Supplemental: Libraries that use the underlying system
  - Testing: Extraneous support for testing the standard libraries

Author: etcwilde

Runtimes/Readme.md

@@ -0,0 +1,81 @@
+# Swift Runtime Libraries
+
+This directory contains the pieces of the Swift runtime libraries.
+
+## Layering
+
+```
+╔═══════════════╗
+║               ║
+║    Testing    ║
+║               ║
+╠───────────────╣
+│               │
+│ Supplemental  │
+│               │
+├───────────────┤
+│               │
+│    Overlay    │
+│               │
+├───────────────┤
+│               │
+│     Core      │
+│               │
+└───────────────┘
+```
+
+### Core
+
+The _Core_ project contains the basic datatypes and underpinnings used by the
+rest of the libraries that make up the standard library. The _Core_ libraries
+must be built first.
+The _Core_ project provides the following libraries:
+ - `swiftCore`
+ - `swift_Concurrency`
+ - `SwiftOnoneSupport`
+ - `CommandLine`
+ - `Demangling`
+ - `Runtime`
+ - `LLVMSupport`
+ - `StdlibStubs`
+ - `Threading`
+ - `SwiftShim`
+
+These libraries must work across the platforms that Swift supports.
+
+### Overlay
+
+The Overlay project contains a few default platform overlay libraries. A
+platform overlay is responsible for exposing the system libraries into Swift in
+an ergonomic fashion. On most systems, this exposes the C standard library and a
+few other libraries that are normally available on that system. The overlay
+libraries are allowed to depend on any of the runtime libraries provided by the
+_Core_ project and libraries distributed by the platform.
+
+The platform overlay is specific to the platform that it overlays and cannot be
+used across platforms.
+
+### Supplemental
+
+The supplemental libraries provide the remainder of the standard distribution of
+libraries provided by Swift.
+
+The Supplemental libraries include:
+ - `RegexParser`
+ - `StringProcessing`
+ - `RegexBuilder`
+ - `Cxx Interop`
+ - `Synchronization`
+ - `Distributed`
+ - `Observation`
+
+The behavior of these libraries may differ slightly based on the behavior of the
+underlying operating system. These libraries are allowed to depend on the
+platform overlay and the libraries provided by the _Core_ project.
+
+### Testing
+
+The testing project provides testing support for the libraries that make up the
+standard library. These libraries are optional and are not intended for
+shipping. They must be built last, after the Core, the Overlay, and the
+Supplemental libraries.