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
1 file changed