add cpp29 badge to readme (#27)

Author: JeffGarland

README.md

@@ -7,6 +7,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 ![Library Status](https://raw.githubusercontent.com/bemanproject/beman/refs/heads/main/images/badges/beman_badge-beman_library_under_development.svg)
 ![Continuous Integration Tests](https://github.com/bemanproject/any_view/actions/workflows/ci_tests.yml/badge.svg)
 ![Lint Check (pre-commit)](https://github.com/bemanproject/any_view/actions/workflows/pre-commit.yml/badge.svg)
+![Standard Target](https://github.com/bemanproject/beman/blob/main/images/badges/cpp29.svg)
 
 **Implements**: `std::ranges::any_view` proposed in [any_view (P3411R2)](https://wg21.link/p3411r2).