Add upgrade guide

UPGRADE.md

@@ -0,0 +1,42 @@
+# Upgrading to 4.0.0
+
+## Constructors Of `FetchServer` and `Server` Take An Object
+
+For `Server`:
+
+```diff
+import { Server } from 'fakerest';
+import { data } from './data';
+
+-const server = new Server('http://myapi.com');
++const server = new Server({ baseUrl: 'http://myapi.com' });
+server.init(data);
+```
+
+For `FetchServer`:
+
+```diff
+import { FetchServer } from 'fakerest';
+import { data } from './data';
+
+-const server = new FetchServer('http://myapi.com');
++const server = new FetchServer({ baseUrl: 'http://myapi.com' });
+server.init(data);
+```
+
+## Constructor Of `Collection` Take An Object
+
+```diff
+-const posts = new Collection([
+-    { id: 1, title: 'baz' },
+-    { id: 2, title: 'biz' },
+-    { id: 3, title: 'boz' },
+-]);
++const posts = new Collection({
++    items: [
++        { id: 1, title: 'baz' },
++        { id: 2, title: 'biz' },
++        { id: 3, title: 'boz' },
++    ],
++});
+```