add requirement of zend-stratigility to documentation

docs/book/middleware.md

@@ -18,6 +18,15 @@ is callable. When those conditions are met, it uses the [PSR-7 bridge](https://g
 to convert the zend-http request and response objects into PSR-7 instances, and
 then invokes the middleware.
 
+Starting with version 3.1.0, the `zendframework/zend-stratigility:^2.0` package is needed if you want to dispatch
+middleware. Be sure to add it to your project:
+
+```bash
+$ composer require zendframework/zend-stratigility:^2.0
+```
+
+Version 3 of `zend-stratigility` is currently not supported.
+
 ## Mapping routes to middleware
 
 The first step is to map a route to PSR-7 middleware. This looks like any other

docs/book/migration/to-v3-0.md

@@ -294,6 +294,16 @@ LogFormatterManager | `log_formatters` | `Zend\Log\Formatter\LogFormatterProvide
 This additions allow you to provide additional plugins for every aspect zend-log
 exposes.
 
+## Middleware
+
+Version 2.7 added support for dispatching middleware. If you use this feature and are upgrading to `zend-mvc` 3.1.0 or
+higher, the [`zendframework/zend-stratigility:^2.0`](https://github.com/zendframework/zend-stratigility) package
+is needed.
+
+```bash
+$ composer require zendframework/zend-stratigility:^2.0
+```
+
 ## Plugins
 
 The following plugins have been removed from the main zend-mvc repository, and