Update the link to migration instructions (#916)

lib/application.js

@@ -107,7 +107,8 @@ module.exports = class Application extends Emitter {
     if (isGeneratorFunction(fn)) {
       deprecate('Support for generators will be removed in v3. ' +
                 'See the documentation for examples of how to convert old middleware ' +
-                'https://github.com/koajs/koa/tree/v2.x#old-signature-middleware-v1x---deprecated');
+                'https://github.com/koajs/koa/blob/master/Readme.md' +
+                '#old-signature-middleware-v1x---deprecated');
       fn = convert(fn);
     }
     debug('use %s', fn._name || fn.name || '-');