README.md: Add crates.io and docs.rs badges
<a href="http://libp2p.io/"><img src="https://img.shields.io/badge/project-libp2p-yellow.svg?style=flat-square" /></a>
[![dependency status](https://deps.rs/repo/github/libp2p/rust-libp2p/status.svg?style=flat-square)](https://deps.rs/repo/github/libp2p/rust-libp2p)
+[![Crates.io](https://img.shields.io/crates/v/libp2p.svg)](https://crates.io/crates/libp2p)
+[![docs.rs](https://img.shields.io/badge/api-rustdoc-blue.svg)](https://docs.rs/libp2p)
This repository is the central place for Rust development of the [libp2p](https://libp2p.io) spec.
