Backport of Update README.md into release/0.14.x

[hc-github-team-secure-boundary]

trafficstars

Backport

This PR is auto-generated from #3734 to be assessed for backporting due to the inclusion of the label backport/website.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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Adding link to "What is Boundary?" in the README

[hashicorp-cla]

All committers have signed the CLA.

[moduli]

I believe the actual intention was to add this to release/0.14.x instead of stable-website. There was another backport PR that handled stable-website.

Manually cherry-picked via https://github.com/hashicorp/boundary/commit/c358b2200a24731fb780af4d0a223975fa4a7abc