Notes when the ContentLengthMiddleware was introduced

Author: weierophinney

doc/book/features/helpers/content-length.md

@@ -1,5 +1,7 @@
 # Content-Length Middleware
 
+- **Available since zend-expressive-helpers version 4.1.0.**
+
 In some cases, you may want to include an explicit `Content-Length` response
 header, without having to inject it manually. To facilitate this, we provide
 `Zend\Expressive\Helper\ContentLengthMiddleware`.

doc/book/features/helpers/intro.md

@@ -7,7 +7,7 @@ may integrate features or simply provide standalone benefits.
 Currently, these include:
 
 - [Body Parsing Middleware](body-parse.md)
-- [Content-Length Middleware](content-length.md)
+- [Content-Length Middleware](content-length.md) (since 4.1.0)
 - [UrlHelper](url-helper.md)
 - [ServerUrlHelper](server-url-helper.md)