added history prop upgrade information

Author: ryanflorence

upgrade-guides/v2.0.0.md

@@ -22,6 +22,34 @@ We have done our best to provide backwards compatibility with deprecated APIs. I
 
 The deprecation warnings should also lead you to the relevant part of this document. If one doesn't, please open a pull request with a fix. Also, if any part of this document could be improved, please let us know how. Confound it, our bias is often inescapable!
 
+## Router `history` prop
+
+### Default Hash History
+
+`Router` used to default to creating a hash history, it no longer creates a default, you must provide one.
+
+```js
+// v1.x
+<Router/>
+
+// v2.0.0
+// hash history
+import { hashHistory } from 'react-router'
+<Router history={hashHistory}/>
+```
+
+### Using Browser History
+
+```js
+// v1.x
+import createBrowserHistory from 'history/lib/createBrowserHistory'
+<Router history={createBrowserHistory()}/>
+
+// v2.0.0
+import { browserHistory } from 'react-router'
+<Router history={browserHistory}/>
+```
+
 ## New Router render prop (`RoutingContext` prop replacement)
 
 You can now pass a `render` prop to `Router` for it to use for rendering. This allows you to create "middleware components" that participate in routing. Its critical for integrations with libraries like Relay, Redux, Resolver, Transmit, Async Props, etc.
@@ -240,4 +268,5 @@ const appHistory = useRouterHistory({
 })
 
 <Router history={appHistory}/>
+```