Help: Add release note for GoogleTest module DISCOVERY_MODE feature

This feature was added by commit 75e82a13db (GoogleTest: Add new
DISCOVERY_MODE option to gtest_discover_tests, 2020-03-16) but a
release note was accidentally left out.  Add one now.

Help/release/dev/GoogleTest-DISCOVERY_MODE.rst

@@ -0,0 +1,7 @@
+GoogleTest-DISCOVERY_MODE
+-------------------------
+
+* The :module:`GoogleTest` module :command:`gtest_discover_tests` command
+  gained a new ``DISCOVERY_MODE`` option to control when the test
+  discovery step is run.  It offers a new ``PRE_TEST`` setting to
+  run the discovery at test time instead of build time.