README: Add more hints about cleanup and reinstallation

[cgwalters]

The stale auth directory was what was breaking my cluster reinstalls.
Let's encourage people to use --dir.

[wking]

Mitigates #522.

[wking]

This is not libvirt-specific. Move it to the README quick-start (which we link here)?

[wking]

@cgwalters, do you think you'll have time to get back to this in the next day or two? Otherwise, can I help somehow (e.g. push a fixup commit up for you to pull, push a fixup straight to your branch, file a replacement PR, ...)?

[cgwalters]

push a fixup straight to your branch,

This option seems good to me, but it's up to you too!

[wking]

I've force-pushed 491ff5a -> fddc4dc to @cgwalters' branch, moving the text to the platform-agnostic README, reformatting to one sentence per line, adding a missing comma, and changing "state directory" -> "asset directory". @cgwalters, does that look good to you?

[cgwalters]

/lgtm

[openshift-ci-robot]

@cgwalters: you cannot LGTM your own PR.

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[wking]

/lgtm
/retest

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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