Update upgrade command with 0.26 breaking changes

Summary:
Generally it feels that at some point we will just integrate `rnpm upgrade` directly here, but this is a super temporary quick workaround for the people upgrading to 0.26. We might want to change it later to 0.27 and update this message in case we have any other stuff to upgrade.

![screen shot 2016-05-18 at 14 28 47](https://cloud.githubusercontent.com/assets/2464966/15358460/df3b1b66-1d04-11e6-825c-fa18fa96ee6e.png)

It is supposed to work for 0.26.0, 0.26.1 and all the others up to 0.27.
Closes https://github.com/facebook/react-native/pull/7614

Differential Revision: D3316420

fbshipit-source-id: 3861b4228ee878464e18ba3de1f3e0c12d5f30d1

local-cli/upgrade/upgrade.js

@@ -57,6 +57,17 @@ module.exports = function upgrade(args, config) {
             );
             return Promise.resolve();
           }
+          
+          if (semver.satisfies(v, '~0.26.0')) {
+            console.log(
+              chalk.yellow(
+                'React Native 0.26 introduced some breaking changes to the native files on iOS. You can\n' +
+                'perform them manually by checking the release notes or use \'rnpm\' to do it automatically.\n' +
+                'Just run:\n' +
+                '\'npm install -g rnpm && npm install rnpm-plugin-upgrade@0.26 --save-dev\', then run \'rnpm upgrade\''
+              )
+            );
+          }
         } else {
           console.log(
             chalk.yellow(