Add changelog for substitute plugin

While rebasing master into this feature branch I removed fdaniele85's
original version(s) of the changelog to make conflict resolvement
easier. This is a slightly extended version of the latest version I
found in the original commits.

docs/changelog.rst

@@ -115,6 +115,10 @@ New features:
 * :doc:`/plugins/embyupdate`: Add handling for private users by adding
   ``userid`` config option.
   :bug:`4402`
+* :doc:`/plugins/substitute`: Add the new plugin `substitute` as an alternative
+  to the `rewrite` plugin. The main difference between them being that
+  `rewrite` modifies files' metadata and `substitute` does not.
+  :bug:`2786`
 
 Bug fixes: