docs: clarify bidirectional Comlink patterns

[KirtiRamchandani]

Problem

• Closes Explicitly mention in the document of Comlink's limitation on bidirectional communication between main thread and worker #672.
• The README documents callbacks and endpoints separately, but does not explicitly explain how to model two-way communication.

Root cause

• wrap() talks to one value exposed by the other side, while expose() makes one local value available. Users can miss that callbacks, proxied objects, or additional endpoints are needed when both sides need to call each other.

Solution

• Add a short README section describing supported bidirectional patterns:
  • pass callbacks or objects with Comlink.proxy()
  • use Comlink.createEndpoint() or a separate MessageChannel for another long-lived proxy
  • avoid assuming wrap() and expose() alone expose both local object graphs

Tests run

• npx prettier@2.8.3 --check README.md
• git diff --check