Adding UPGRADE_GUIDE

UPGRADE_GUIDE.md

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+# Upgrade Guide
+
+### 0.5.x -> 0.6.0
+
+The `0.6.0` release contains mostly bug fixes, but there are a couple things to be aware of when upgrading.
+
+#### ES6 Promise Polyfill
+
+Up until the `0.6.0` release ES6 `Promise` was being polyfilled using [es6-promise](https://github.com/jakearchibald/es6-promise). With this release, the polyfill has been removed, and you will need to supply the it yourself if your environment needs it.
+
+```js
+require('es6-promise').polyfill();
+var axios = require('axios');
+```
+
+This will polyfill the global environment, and only needs to be done once.
+
+#### `axios.success`/`axios.error`
+
+The `success`, and `error` aliases were deprectated in [0.4.0](https://github.com/mzabriskie/axios/blob/master/CHANGELOG.md#040-oct-03-2014). As of this release they have been removed entirely. Instead please use `axios.then`, and `axios.catch` respectively.
+
+```js
+axios.get('some/url')
+  .then(function (res) {
+    /* ... */
+  })
+  .catch(function (err) {
+    /* ... */
+  });
+```
+
+#### UMD
+
+Previous versions of axios shipped with an AMD, CommonJS, and Global build. This has all been rolled into a single UMD build.
+
+```js
+// AMD
+require(['bower_components/axios/dist/axios'], function (axios) {
+  /* ... */
+});
+
+// CommonJS
+var axios = require('axios/dist/axios');
+```