docs: add warning about js-sdk security

[thephez]

Issue being fixed or feature implemented

Since this SDK doesn't support proofs we need a warning to make sure that is obvious

Replaces #2106 which couldn't complete CI

What was done?

Add warning to top of README

How Has This Been Tested?

N/A

Breaking Changes

N/A

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone