Add a minimum rustc badge

Author: kngwyu

README.md

@@ -4,6 +4,7 @@
 [![Actions Status](https://github.com/PyO3/pyo3/workflows/Test/badge.svg)](https://github.com/PyO3/pyo3/actions)
 [![codecov](https://codecov.io/gh/PyO3/pyo3/branch/master/graph/badge.svg)](https://codecov.io/gh/PyO3/pyo3)
 [![crates.io](http://meritbadge.herokuapp.com/pyo3)](https://crates.io/crates/pyo3)
+[![minimum rustc 1.42](https://img.shields.io/badge/rustc-1.42+-blue.svg)](https://rust-lang.github.io/rfcs/2495-min-rust-version.html)
 [![Join the dev chat](https://img.shields.io/gitter/room/nwjs/nw.js.svg)](https://gitter.im/PyO3/Lobby)
 
 [Rust](http://www.rust-lang.org/) bindings for [Python](https://www.python.org/). This includes running and interacting with Python code from a Rust binary, as well as writing native Python modules.
@@ -16,7 +17,7 @@ A comparison with rust-cpython can be found [in the guide](https://pyo3.rs/maste
 
 ## Usage
 
-PyO3 supports Python 3.5 and up. The minimum required Rust version is 1.44.
+PyO3 supports Python 3.5 and up. The minimum required Rust version is 1.42.
 
 PyPy is also supported (via cpyext) for Python 3.5 only, targeted PyPy version is 7.0.0.
 Please refer to the [pypy section in the guide](https://pyo3.rs/master/pypy.html).