Update README to use repo.

[robholland]

What was changed

Switch to documenting install via the helm repo by default.

Why?

The helm repo will always give a release version, which will be more thoroughly tested than what happens to be in main when the user clones the repo.

Checklist

1. Closes #458

2. How was this tested:

3. Any docs updates needed?

[CLAassistant]

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1 out of 2 committers have signed the CLA.

:white_check_mark: robholland
:x: axfelix
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[sahilvv]

/LGTM