Update README for using ably-js in Next.js 15

[VeskeR]

Something has changed in Next.js 15 so it's now necessary to load components using ably-js with next/dynamic and use 'use client'; directive for such components, otherwise we would get Warning: Invalid hook call. Hooks can only be called inside of the body of a function component. This could happen for one of the following reasons: errors and possible build issues like in #1788.

See working Next.js 15 (RC) example app: https://github.com/VeskeR/ably-js-nextjs-15-example (need to run npm install --legacy-peer-deps due to this npm issue and using prerelease version of next.js).
And see Next.js 14 app: https://github.com/ably-labs/ably-nextjs-fundamentals-kit. You can remove 'use client'; there and skip loading components using next/dynamic and it would still work.

Need to investigate what's changed and update README for using ably-js with Next.js 15.

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