add macos-11 runner with Clang 13 (#893)

Author: DyXel

.github/workflows/build-cppfront.yaml

@@ -27,6 +27,9 @@ jobs:
           - compiler: clang++-14
             cxx-std: 'c++2b'
         include:
+          - runs-on: macos-11
+            compiler: clang++
+            cxx-std: 'c++20'
           - runs-on: macos-latest
             compiler: clang++
             cxx-std: 'c++20'

README.md

@@ -5,6 +5,7 @@
 See [License](LICENSE)
 
 [![Contributor Covenant](https://img.shields.io/badge/Contributor%20Covenant-2.1-4baaaa.svg)](CODE_OF_CONDUCT.md)
+[![Build (clang, gcc, vs)](https://github.com/hsutter/cppfront/actions/workflows/build-cppfront.yaml/badge.svg)](https://github.com/hsutter/cppfront/actions/workflows/build-cppfront.yaml)
 
 Cppfront is an experimental compiler from a potential C++ 'syntax 2' (Cpp2) to today's 'syntax 1' (Cpp1), to learn some things, prove out some concepts, and share some ideas. This compiler is a work in progress and currently hilariously incomplete... basic functions work, classes will be next, then metaclasses and lightweight exceptions.