Add build badge for link-checking

Author: claremacrae

README.md

@@ -1,5 +1,7 @@
 # IncludeCpp Website
 
+[![Check Markdown links](https://github.com/include-cpp/include-cpp.github.io/actions/workflows/action.yml/badge.svg)](https://github.com/include-cpp/include-cpp.github.io/actions/workflows/action.yml)
+
 This site is published at [https://www.includecpp.org/](https://www.includecpp.org/).
 
 ## Help and instructions for maintaining this site