Added stub release notes for 2.2.13.

docs/releases/2.2.13.txt

@@ -0,0 +1,12 @@
+===========================
+Django 2.2.13 release notes
+===========================
+
+*Expected June 1, 2020*
+
+Django 2.2.13 fixes a bug in 2.2.12.
+
+Bugfixes
+========
+
+* ...

docs/releases/index.txt

@@ -53,6 +53,7 @@ versions of the documentation contain the release notes for any later releases.
 .. toctree::
    :maxdepth: 1
 
+   2.2.13
    2.2.12
    2.2.11
    2.2.10