Update readme to clarify toolchain version.

Author: rxwei

README.md

@@ -1,8 +1,11 @@
-# MatchingEngine
+# Declarative String Processing for Swift
 
 An early experimental general-purpose pattern matching engine for Swift.
 
 See [Declarative String Processing Overview][decl-string]
 
 [decl-string]: Documentation/DeclarativeStringProcessing.md
 
+## Requirements
+
+- [Swift Trunk Development Snapshot](https://www.swift.org/download/#snapshots) DEVELOPMENT-SNAPSHOT-2022-02-03 or later.