CONVENTIONS.md: add runlevel documentation

[mcoops]

Brief summary on what runlevels are, some reasons why they have been historically required and why the defining of the runlevel label is no longer needed.

Based on documentation worked on with @deads2k - with the help greatly appreciated.

Also addresses #303

[openshift-ci]

Hi @mcoops. Thanks for your PR.

I'm waiting for a openshift member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[dhellmann]

@mcoops, if you make your membership in the openshift org on Github public the bot will be able to see that it's OK to test your PRs.

/ok-to-test

[mcoops]

@dhellmann is there anything else I should be doing to help this PR along?

[dhellmann]

@mcoops the text you've written is clear to me, but I'm not a subject matter expert so I don't think it's a good idea for me to approve it. Let me see if I can help chase down a reviewer for you.

[deads2k]

The text is accurate. Thanks!

/lgtm

[dhellmann]

/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

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