Help: Add notes for topic 'update-curl'

Help/release/dev/update-curl.rst

@@ -0,0 +1,6 @@
+update-curl
+-----------
+
+* The version of curl bundled with CMake no longer accepts URLs of the form
+  ``file://c:/...`` on Windows due to a change in upstream curl 7.52.  Use
+  the form ``file:///c:/...`` instead to work on all versions.