docs: clarify reference implementation permalink behavior

[ChrisWozniak]

Fixes #121.

Summary

Clarifies that go-algorand reference-implementation links are commit-pinned permalinks. They provide stable references but are not guaranteed to point to the latest revision.

Validation

• I ran make ci, or make check with no version-drift warnings. Docker Desktop is not running, and the native pinned mdBook toolchain is unavailable.
• I checked the rendering with make serve, or make docker-serve. Not run; this is a prose-only change with no rendering-specific syntax.
• rumdl, end-of-file, and trailing-whitespace hooks passed for src/READING-GUIDELINES.md using Python 3.14.4. The repository pins Python 3.14.6, which is unavailable to the installed uvx.
• The local-link hook could not execute under the Windows/WSL environment because the hook received a malformed Windows path. No links were added or changed.