Add CI badge to README

Author: Barrett Adair

README.md

@@ -4,7 +4,7 @@ Distributed under the Boost Software License, Version 1.0.
 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
 -->
 
-# Boost.CallableTraits
+# Boost.CallableTraits <a target="_blank" href="https://github.com/boostorg/callable_traits/actions/workflows/ci.yml">![CI][badge.CI]</a>
 
 CallableTraits is a standalone C++11 header-only library for the inspection, synthesis, and decomposition of callable types. Language features added in later C++ standards are also supported.
 
@@ -14,3 +14,5 @@ CallableTraits is released as part of the [Boost C++ Libraries](http://www.boost
 
 Licensed under the [Boost Software License, Version 1.0](LICENSE.md).
 
+<!-- Links -->
+[badge.CI]: https://github.com/boostorg/callable_traits/actions/workflows/ci.yml/badge.svg