Add resource type tags guidance

[fengfeng-zi]

Summary

• add a small resource-tag legend near the top of the README
• document a single resource-type tag rule in CONTRIBUTING.md
• tag a few existing C/C++ entries as a pilot example instead of retagging the whole list

Why this scope

• addresses Add resource type alongside tutorial title. #63 with a newcomer-friendly change
• keeps the taxonomy intentionally small: [guide], [video], [course], [book], [repo]
• avoids a noisy full-file retagging pass in the first PR

Closes #63