Add CHANGELOG for invocation-scoped fixtures

Author: nicoddemus

CHANGELOG.rst

@@ -116,7 +116,11 @@
   Example '-o xfail_strict=True'. A complete ini-options can be viewed
   by py.test --help. Thanks `@blueyed`_ and `@fengxx`_ for the PR
 
-*
+* New scope for fixtures: ``"invocation"``. This fixtures may be requested by fixtures from
+  any scope, when they assume the same scope as the fixture requesting it. An ``invocation``-scoped
+  fixture can be requested from different scopes in the same test session,
+  in which case each scope will have its own copy. This feature is considered experimental.
+  Thanks `@nicoddemus`_ for the PR.
 
 *