README update

Author: paulb777

README.md

@@ -1,3 +1,12 @@
+# Swift Package Manager copy repository
+
+This repository is a copy of https://github.com/firebase/abseil-cpp with
+the git metadata removed to provide one to two orders of magnitude increase in
+speed for Swift Package Manager usage.
+
+There should be no changes to this repo other than updates from its mirror
+and Swift Package Manager specific items.
+
 # Abseil - C++ Common Libraries
 
 The repository contains the Abseil C++ library code. Abseil is an open-source