Add warning banners to legacy services (#2546)

services/buildkite/buildkite.service.js

@@ -4,6 +4,12 @@ const LegacyService = require('../legacy-service')
 const { makeBadgeData: getBadgeData } = require('../../lib/badge-data')
 const { checkErrorResponse } = require('../../lib/error-helper')
 
+// This legacy service should be rewritten to use e.g. BaseJsonService.
+//
+// Tips for rewriting:
+// https://github.com/badges/shields/blob/master/doc/rewriting-services.md
+//
+// Do not base new services on this code.
 module.exports = class Buildkite extends LegacyService {
   static get category() {
     return 'build'