Add codecov.io badge on the README

Signed-off-by: Thibaut Vandervelden <thvdveld@vub.be>

README.md

@@ -4,6 +4,7 @@
 [![crates.io](https://img.shields.io/crates/v/smoltcp.svg)](https://crates.io/crates/smoltcp)
 [![crates.io](https://img.shields.io/crates/d/smoltcp.svg)](https://crates.io/crates/smoltcp)
 [![crates.io](https://img.shields.io/matrix/smoltcp:matrix.org)](https://matrix.to/#/#smoltcp:matrix.org)
+[![codecov](https://codecov.io/github/smoltcp-rs/smoltcp/branch/master/graph/badge.svg?token=3KbAR9xH1t)](https://codecov.io/github/smoltcp-rs/smoltcp)
 
 _smoltcp_ is a standalone, event-driven TCP/IP stack that is designed for bare-metal,
 real-time systems. Its design goals are simplicity and robustness. Its design anti-goals