add deprecation notice for $highlight-yellow

Author: shawnbot

deprecations.js

@@ -13,6 +13,10 @@ const versionDeprecations = {
       variables: ['$status-pending'],
       message: `This variable is deprecated.`
     },
+    {
+      variables: ['$highlight-yellow'],
+      message: `This variable is deprecated.`
+    },
     {
       variables: ['$repo-private-text', '$repo-private-bg', '$repo-private-icon'],
       message: `These variables are deprecated.`