Add teaching skill and related documentation for enhanced learning experience

skills/productivity/README.md

@@ -5,4 +5,5 @@ General workflow tools, not code-specific.
 - **[caveman](./caveman/SKILL.md)** — Ultra-compressed communication mode. Cuts token usage ~75% by dropping filler while keeping full technical accuracy.
 - **[grill-me](./grill-me/SKILL.md)** — Get relentlessly interviewed about a plan or design until every branch of the decision tree is resolved.
 - **[handoff](./handoff/SKILL.md)** — Compact the current conversation into a handoff document so another agent can continue the work.
+- **[teach](./teach/SKILL.md)** — Teach the user a new skill or concept over multiple sessions, using the current directory as a stateful teaching workspace.
 - **[write-a-skill](./write-a-skill/SKILL.md)** — Create new skills with proper structure, progressive disclosure, and bundled resources.