Highlighting piece of code covered by GitHub Actions workflow

Added comment highlighting piece of CircleCI config code for which
a GitHub Actions workflow has been already created (as part of work on
RFC-18 that requires migration of build processes from CircleCI to
GitHub Actions).

.circleci/config.yml

@@ -36,7 +36,7 @@ jobs:
           root: /tmp/docs
           paths:
             - solidity/*
-  build_client_and_test_go:
+  build_client_and_test_go: # GitHub Actions equivalent of this job (.github/workflows/client.yml) has been created as part of work on RFC-18
     executor: docker-git
     steps:
       - setup_remote_docker: