Document missing ingress.className value in README.md

[c0bra]

Pull Request

What does this PR do?

The README.md is missing documentation of the ingress.className value. If you mistakenly use an annotation like kubernetes.io/ingress.class: nginx you will get an error and scratch your head about how to address it.

PR checklist

Please check if your PR fulfills the following requirements:

• [X] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• [X] Have you read the contributing guidelines?
• [X] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!

[c0bra]

@alallema sure, done!

[alallema]

bors merge

[bors[bot]]

Build succeeded:

• check-chart-manifest-in-sync
• helm-tests
• linter