Add C++23 to supported standards list (#851)

diff --git a/README.md b/README.md
index 5f389a8..cfa337c 100644
--- a/README.md
+++ b/README.md
@@ -29,7 +29,7 @@
 
 **doctest** is a new C++ testing framework but is by far the fastest both in compile times (by [**orders of magnitude**](doc/markdown/benchmarks.md)) and runtime compared to other feature-rich alternatives. It brings the ability of compiled languages such as [**D**](https://dlang.org/spec/unittest.html) / [**Rust**](https://doc.rust-lang.org/book/second-edition/ch11-00-testing.html) / [**Nim**](https://nim-lang.org/docs/unittest.html) to have tests written directly in the production code thanks to a fast, transparent and flexible test runner with a clean interface.
 
-[![Standard](https://img.shields.io/badge/c%2B%2B-11/14/17/20-blue.svg)](https://en.wikipedia.org/wiki/C%2B%2B#Standardization)
+[![Standard](https://img.shields.io/badge/c%2B%2B-11/14/17/20/23-blue.svg)](https://en.wikipedia.org/wiki/C%2B%2B#Standardization)
 [![License](https://img.shields.io/badge/license-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 [![download](https://img.shields.io/badge/download%20%20-link-blue.svg)](https://raw.githubusercontent.com/doctest/doctest/master/doctest/doctest.h)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/503/badge)](https://bestpractices.coreinfrastructure.org/projects/503)