Docs: add migration guide for v5

Explain how to update to use the default sandbox

docs/guides/migrating-to-5.0.md

@@ -0,0 +1,47 @@
+---
+layout: page
+title: Migrating to v5.0 - Sinon.JS
+breadcrumb: migrating to 5.0
+---
+
+As with all `MAJOR` releases in [`semver`](http://semver.org/), there are breaking changes in `sinon@5`.
+This guide will walk you through those changes.
+
+## `sinon` is now a (default) sandbox
+
+Since `sinon@5.0.0`, the `sinon` object is a default sandbox. Unless you have a very advanced setup or need a special configuration, you probably want to just use that one.
+
+The old sandbox API is still available, so you don't **have** to do anything.
+
+However, switching to using the default sandbox can help make your code more concise.
+Now would be a good opportunity to tidy up your tests.
+
+
+### Before `sinon@5.0.0`
+
+In earlier versions, you would manually have to create sandboxes and keep references to them in order to restore them.
+
+```js
+const sandbox = sinon.createSandbox();
+
+describe('myFunction', function() {
+    afterEach(function () {
+        sandbox.restore();
+    });
+
+    it('should make pie');
+});
+```
+
+
+### From `sinon@5.0.0`
+
+```js
+describe('myFunction', function() {
+    afterEach(function () {
+        sinon.restore();
+    });
+
+    it('should make pie');
+});
+```