Update readme with supported backends.

[jeffmendoza]

Description of the PR

Clarify and highlight the supported backends.

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If OpenAPI spec is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged

[jeffmendoza]

cc @funnelfiasco for review.

[funnelfiasco]

Looks good generally, although I think my suggested wording tweak in the ent entry is worth adopting.

I have some proto-thoughts about how we might move backends (and other feature additions) through the lifecycle, but they're not nearly well-formed enough to block this PR. Something to figure out with the community over time.

[jeffmendoza]

I have some proto-thoughts about how we might move backends (and other feature additions) through the lifecycle, but they're not nearly well-formed enough to block this PR. Something to figure out with the community over time.

Looking forward to it.

[mihaimaruseac]

Thank you