docs: add two-way communication guidance

[KirtiRamchandani]

Problem

#672 asks for explicit documentation around using Comlink when both sides of a channel need to call into each other.

Root cause

The README documents callbacks and one-direction worker APIs, but does not spell out how wrap() and expose() compose when each side has its own API. That can make users infer an unsupported limitation or try to overload one endpoint with multiple objects.

Solution

Add a Two-way communication section that:

• explains that wrap() consumes the other side while expose() publishes the current side,
• shows a main-thread/worker example where both sides expose APIs,
• points callback-style use cases to Comlink.proxy(), and
• suggests MessageChannel/createEndpoint() for independent APIs.

Tests run

• npm exec --yes prettier@2.8.3 -- --check README.md
• git diff --check

Fixes #672