Add a section to UPGRADING.md describing the new DELETE status code behaviour and how it’s changed over the past few versions

Author: jthornec

UPGRADING.md

@@ -1,6 +1,36 @@
 Upgrading Grape
 ===============
 
+### Upgrading to >= 0.19.1 (next)
+
+#### DELETE now defaults to status code 200 for responses with a body, or 204 otherwise
+
+DELETE responses now use a 200 status code by default when a body is present, or 204 otherwise.
+
+- In versions < 0.19.0, all DELETE requests defaulted to a 200 OK status code.
+- In version 0.19.0, all DELETE requests defaulted to a 204 No Content status code, even when content was included in the response.
+- As of version 0.19.1, DELETE requests default to a 204 No Content status code, unless content is supplied, in which case they default to a 200 OK status code.
+
+To achieve the old behavior, one can specify the status code explicitly:
+
+```ruby
+delete :id do
+  status 204 # or 200, for < 0.19.0 behavior
+  'foo successfully deleted'
+end
+```
+
+One can also use the new `return_no_content` helper to explicitly return 204 and an empty body for any request type:
+
+```ruby
+delete :id do
+  return_no_content
+  'this will not be returned'
+end
+```
+
+See [#1550](https://github.com/ruby-grape/grape/pull/1550) for more information.
+
 ### Upgrading to >= 0.18.1
 
 #### Changes in priority of :any routes