Add shields

README.md

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 Unwinding library in Rust and for Rust
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+[![crates.io](https://img.shields.io/crates/v/unwinding.svg)](https://crates.io/crates/unwinding)
+[![docs.rs](https://docs.rs/unwinding/badge.svg)](https://docs.rs/unwinding)
+[![license](https://img.shields.io/crates/l/unwinding.svg)](https://crates.io/crates/unwinding)
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 This library serves two purposes:
 1. Provide a pure Rust alternative to libgcc_eh or libunwind.
 2. Provide easier unwinding support for `#![no_std]` targets.