Add concise DISCLAIMER for migrate2rocky.sh

[tcooper]

I propose we add a single, clear DISCLAIMER to this directory in the form of a separate file which can be included / linked in the README and from other sources (eg. Technical Notes / Guides / Documentation).

The full DISCLAIMER.md should be shown if/when migrate2rocky.sh is run with the -h option and a short DISCLAIMER referencing the full DISCLAIMER.md (via a link and/or suggestion to run with -h) should be shown before a migration is attempted.

[pajamian]

I like this idea, can you do up a PR for this?