docs: add usage sub-heading to readme

[MikeMcC399]

Issue

In the README section Tag Selection there is a disclaimer-type section with no heading and which does not have anything to do with tag selection. So it is mixed with a section where it does not belong and, because there is no separate heading, it can't easily be referred to.

Change

• The text was originally added by @jaffrepaul through PR #830 which refers to "Image security".

Add the neutral heading "Usage". If the Cypress.io team prefers a different term, please add review feedback and I will change it accordingly.

The paragraphs are raised from quoted text to regular text.

[MikeMcC399]

Preview

https://github.com/MikeMcC399/cypress-docker-images/blob/image-security/README.md#usage

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[MikeMcC399]

@jennifer-shehane

I would like to close off some stale vulnerability reports and I also wanted to have a link to use!