Add readme

Author: Manishearth

README.md

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+# unicode-security
+
+[![Build Status](https://github.com/unicode-rs/unicode-security/workflows/Tests/badge.svg)](https://github.com/unicode-rs/unicode-security/actions)
+[![Current Version](https://meritbadge.herokuapp.com/unicode-security)](https://crates.io/crates/unicode-security)
+[![License: MIT/Apache-2.0](https://img.shields.io/crates/l/unicode-security.svg)](#license)
+
+This crate exposes various utilities from [UAX #39 Unicode Security Mechanisms](https://www.unicode.org/reports/tr39/)