ExternalProject: Add missing release note for git update strategy

This release note was mistakenly omitted from commit 0aea435aa1
(ExternalProject: Provide choice of git update strategies, 2020-02-12).

Help/release/dev/fc-ep-git-update-strategy.rst

@@ -0,0 +1,9 @@
+fc-ep-git-update-strategy
+-------------------------
+
+* The :command:`ExternalProject_Add` command gained a new
+  ``GIT_REMOTE_UPDATE_STRATEGY`` keyword.  This can be used to specify how
+  failed rebase operations during a git update should be handled.
+  The ``CMAKE_EP_GIT_REMOTE_UPDATE_STRATEGY`` variable was also added as a
+  global default and is honored by both the :module:`ExternalProject` and
+  :module:`FetchContent` modules.