Add basic README

Author: tkremenek

README.md

@@ -0,0 +1,13 @@
+swift-tools-support-core
+=========================
+
+Contains common infrastructural code for both [SwiftPM](https://github.com/apple/swift-package-manager)
+and [llbuild](https://github.com/apple/swift-llbuild).
+
+License
+-------
+
+Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors.
+Licensed under Apache License v2.0 with Runtime Library Exception.
+
+See http://swift.org/LICENSE.txt for license information.