docs: Add link to rustdocs for `master` to README
I added a link to the master docs to the [README](README.md).
[![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)
+[![docs.rs master](https://img.shields.io/badge/docs-master-blueviolet)](https://libp2p.github.io/rust-libp2p/libp2p/)
This repository is the central place for Rust development of the [libp2p](https://libp2p.io) spec.
