chore(readme): add macOS arm64 to platform support

[n1ckoates]

Please describe the changes this PR makes and why it should be merged: This PR adds "macOS arm64" to the list of supported platforms in README.md. Support was added in https://github.com/discordjs/opus/pull/97 and released in v0.6.0.

Semantic versioning classification:

• [ ] This PR changes the library's interface (methods or parameters added)
  • [ ] This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• [x] This PR only includes non-code changes, like changes to documentation, README, etc.

[n1ckoates]

Also, for future reference, should this be chore(readme) or docs(readme)?

[kyranet]

docs(readme) is fine.