Add a note on experimental jobs

With I4161e1f1c8d47070dd35fad38b00715438d94eb2 and
I37fe007dc6f387d43cbaf55771027718005ac40d we have removed most of the
legacy experimental jobs.

Add a short note so we don't go back to the Hotel California model of
"you can check in but never check out".  With zuulv3 it is possible to
have self-testing changes for testing indiviual jobs.  Experimental
should be for things with wider application to run against all
changes.

Change-Id: Ibfb902b17a8f7d5355689f2a584c061c001df0d8
diff --git a/.zuul.yaml b/.zuul.yaml
index dc3a429..2a2648e 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -323,6 +323,12 @@
       jobs:
         - devstack
         - devstack-unit-tests
+    # Please add a note on each job and conditions for the job not
+    # being experimental any more, so we can keep this list somewhat
+    # pruned.
+    #
+    # * nova-next: maintained by nova for unreleased/undefaulted
+    #    things like cellsv2 and placement-api
     experimental:
       jobs:
         - nova-next