add note about unmount prevention

docs/v1.x-migration.md

@@ -15,6 +15,7 @@
 - [`m.route` and anchor tags](#mroute-and-anchor-tags)
 - [Reading/writing the current route](#readingwriting-the-current-route)
 - [Accessing route params](#accessing-route-params)
+- [Preventing unmounting](#preventing-unmounting)
 - [`m.request`](#mrequest)
 - [`xlink` namespace required](#xlink-namespace-required)
 - [Nested arrays in views](#nested-arrays-in-views)
@@ -408,6 +409,37 @@ m.route(document.body, "/booga", {
 
 ---
 
+## Preventing unmounting
+
+It is no longer possible to prevent unmounting via `onunload`'s `e.preventDefault()`. Instead you should explicitly call `m.route.set` when the expected conditions are met.
+
+### `v0.2.x`
+
+```javascript
+var Component = {
+	controller: function() {
+		this.onunload = function(e) {
+			if (condition) e.preventDefault()
+		}
+	},
+	view: function() {
+		return m("a[href=/]", {config: m.route})
+	}
+}
+```
+
+### `v1.x`
+
+```javascript
+var Component = {
+	view: function() {
+		return m("a", {onclick: function() {if (!condition) m.route.set("/")}})
+	}
+}
+```
+
+---
+
 ## m.request
 
 Promises returned by [m.request](request.md) are no longer `m.prop` getter-setters. In addition, `initialValue` is no longer a supported option.