mcp: polish package doc for v0.3.0

Do a pass through the package doc for v0.3.0.

README for v0.2.0 (#118)

Preliminary readme for v0.2.0.

We will make a GH "release" with the release notes.

mcp: add Session.ID (#67)

Support retrieving the session ID from client and server sessions.

Fixes #65.