Adds CHANGELOG for #270

Author: weierophinney

CHANGELOG.md

@@ -10,7 +10,23 @@ All notable changes to this project will be documented in this file, in reverse
 
 ### Changed
 
-- Nothing.
+- [#270](https://github.com/zendframework/zend-diactoros/pull/270) changes the
+  behavior of `Zend\Diactoros\Server`: it no longer creates an output buffer.
+
+- [#270](https://github.com/zendframework/zend-diactoros/pull/270) changes the
+  behavior of the two SAPI emitters in two backwards-incompatible ways:
+
+  - They no longer auto-inject a `Content-Length` header. If you need this
+    functionality, zendframework/zend-expressive-helpers 4.1+ provides it via
+    `Zend\Expressive\Helper\ContentLengthMiddleware`.
+
+  - They no longer flush the output buffer. Instead, if headers have been sent,
+    or the output buffer exists and has a non-zero length, the emitters raise an
+    exception, as mixed PSR-7/output buffer content creates a blocking issue.
+    If you are emitting content via `echo`, `print`, `var_dump`, etc., or not
+    catching PHP errors or exceptions, you will need to either fix your
+    application to always work with a PSR-7 response, or provide your own
+    emitters that allow mixed output mechanisms.
 
 ### Deprecated