Add warning banners to legacy services (#2546)

services/coveralls/coveralls.service.js

@@ -6,6 +6,12 @@ const {
   coveragePercentage: coveragePercentageColor,
 } = require('../../lib/color-formatters')
 
+// 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 Coveralls extends LegacyService {
   static get category() {
     return 'quality'