add readme to test/support (#68450)

Signed-off-by: Rick Elrod <rick@elrod.me>

test/support/README.md

@@ -0,0 +1,11 @@
+# IMPORTANT!
+
+Files under this directory are not actual plugins and modules used by Ansible
+and as such should **not be modified**. They are used for testing purposes
+only (and are temporary).
+
+In almost every case, pull requests affecting files under this directory
+will be closed.
+
+**You are likely looking for something under
+https://github.com/ansible-collections/ instead.**