docs: clarify linux arm64 install fallback

[haosenwang1018]

Summary

• document the Linux arm64 release-asset gap in the README install section
• improve install.sh download failure output for Linux aarch64
• point affected users to the Bun/source install fallback

Problem

Issue #24 reports that the release installer fails on Linux arm64 because the script looks for mcp-cli-linux-arm64 and gets a 404 when that asset is missing from the latest release. The current error is generic and does not tell users what to do next.

Changes

• add a README note explaining the current Linux arm64 release-binary limitation
• update install.sh to print the missing asset URL and a source-install fallback when the failing platform is Linux aarch64
• keep the existing release-based install path unchanged for platforms with published binaries

Fixes #24

Validation

• bash -n install.sh
• git diff --check