Updated migrating doc from v2/v3 to v4 (remix-run#6198)

* Updated migrating doc from v2/v3 to v4

Added:
- optional parameters
- routeParams
- query - gone, only unparsed search
- getParamNames

* Fix some grammar and wording

Author: vovkvlad

packages/react-router/docs/guides/migrating.md

@@ -10,6 +10,8 @@ React Router v4 is a complete rewrite, so there is not a simple migration path.
 * [Routes](#routes)
   * [Nesting Routes](#nesting-routes)
   * [on* properties](#on-properties)
+  * [Optional Parameters](#optional-parameters)
+  * [Query Strings](#query-strings)
   * [Switch](#switch)
   * [Redirect](#redirect)
 * [PatternUtils](#patternutils)
@@ -121,6 +123,20 @@ React Router v3 provides `onEnter`, `onUpdate`, and `onLeave` methods. These wer
 
 With v4, you should use the lifecycle methods of the component rendered by a `<Route>`. Instead of `onEnter`, you would use `componentDidMount` or `componentWillMount`. Where you would use `onUpdate`, you can use `componentDidUpdate` or `componentWillUpdate` (or possibly `componentWillReceiveProps`). `onLeave` can be replaced with `componentWillUnmount`.
 
+### Optional Parameters
+
+In v3, parameters were made optional with parentheses: `path="/entity/:entityId(/:parentId)"`
+
+In v4, the syntax is changed to a trailing question mark: `path="/entity/:entityId/:parentId?"`
+
+### Query Strings
+
+In v4, there is no parsing done on query strings. The unparsed query string is available on the `location.search` property.
+
+The [qhistory](https://github.com/pshrmn/qhistory) library can provide this functionality if it is necessary for your application.
+
+Read more regarding intentions for this change and possible solutions in [this issue](https://github.com/ReactTraining/react-router/issues/4410).
+
 ### `<Switch>`
 
 In v3, you could specify a number of child routes, and only the first one that matched would be rendered.
@@ -210,7 +226,7 @@ const THING_PATH = '/thing/:id';
 <Link to={PatternUtils.formatPattern(THING_PATH, {id: 1})}>A thing</Link>
 ```
 
-In v4, you can achieve the same functionality using the [compile](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0#compile-reverse-path-to-regexp) function in [`path-to-regexp@^1.7.0`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0).
+In v4, you can achieve the same functionality using the [`compile`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0#compile-reverse-path-to-regexp) function in [`path-to-regexp@^1.7.0`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0).
 
 ```jsx
 // v4
@@ -221,6 +237,10 @@ const thingPath = pathToRegexp.compile(THING_PATH);
 <Link to={thingPath({id: 1})}>A thing</Link>
 ```
 
+### getParamNames
+
+The `getParamNames` functionality can be achieved using the [`parse`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0#parse) function in [`path-to-regexp@^1.7.0`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0).
+
 ## Link
 
 ### `to` property is required