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

[tuvtran]

Thanks for the thoughtful proposal and for keeping the scope small. The list already uses an inline '(video)' marker as its one convention, and adopting a whole new tag taxonomy while only a handful of entries carry tags would leave the README inconsistent and make the contribution rules harder to follow. I'd rather keep the current lightweight format, so I'm closing this — but I appreciate you taking a swing at #63.