Clarify package install commands and release-channel guidance in README

[Copilot]

The README mixed stable and development package context, which made install guidance ambiguous. This update clarifies which package users should install and distinguishes stable usage from main branch development docs.

• Release-channel clarification

  • Added a top-level note that this README primarily describes the stable y-websocket release with yjs v13.
  • Documented that main tracks unstable @y/websocket development for Yjs v14 (@y/y), and that most users should continue using stable v13.

• Install command clarity

  • Updated the install section to show both package names with explicit intent:
    • stable/recommended: y-websocket
    • unstable/main-branch line: @y/websocket

# stable release (recommended)
npm i y-websocket

# main branch / unstable release
npm i @y/websocket