Add warning banners to legacy services (#2546)

services/github/github-release-date.service.js

@@ -10,6 +10,12 @@ const { formatDate } = require('../../lib/text-formatters')
 const { age } = require('../../lib/color-formatters')
 const { documentation } = require('./github-helpers')
 
+// 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 GithubReleaseDate extends LegacyService {
   static get category() {
     return 'activity'