Clarify in README that Policy configuration is not a CRD

[Zalastax]

There have been multiple issues reported that there is no DeschedulerPolicy CRD.
As explained by @a7i in #1351 "The Policy config isn't a k8s resource (so you don't create it as a CRD). It is just a literal text file that you pass to the descheduler binary when it runs."
But this is difficult to understand from the README file and @damemi suggested "maybe we should put a small note in the readme to clarify that this is not a CRD".

It would be interesting for me to see an explanation of this design choice - looking like a CustomResource but mounted as a ConfigMap - but I see that as a bonus.

Cheers!