Update README.md to reflect the addition of PriorityClass

[khalilswdp]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Updated the image on README.md to include PriorityClass, as well as added PriorityClass as a resource that we can simulate.

Which issue(s) this PR fixes:

This PR is related to #27

Special notes for your reviewer:

If there are other changes that I need to make, please tell me and I'll do them.

/label tide/merge-method-squash

[sanposhiho]

Looks good. Please add priority class here as well.
https://github.com/kubernetes-sigs/kube-scheduler-simulator/blob/master/docs/how-it-works.md#1-users-request-creating-resource

[khalilswdp]

I have updated how-it-works

[sanposhiho]

Thanks for the great work @khalilswdp
We finally complete supporting PritorityClass 🎉

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: khalilswdp, sanposhiho

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[khalilswdp]

Thank you @sanposhiho for helping me throughout my first ever contribution to the open source! This is one of the happiest days in my life!