Add course entrypoint

COURSE.md

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+# MiniMind Source Course
+
+This repository is my learning fork of MiniMind.
+
+Original project:
+
+- Repository: https://github.com/jingyaogong/minimind
+- License: Apache-2.0
+
+My main additions are under `course/`:
+
+- `course/00_syllabus.md`: course map and learning route.
+- `course/01_*.md` to `course/28_*.md`: source-code-driven lessons.
+- `course/labs/`: runnable tracing experiments for tokens, labels, shapes, losses, logprobs, rewards, and deployment surfaces.
+- `course/impl/`: teaching implementation of selected MiniMind core ideas.
+- `course/notes/`: personal notes and recurring mistakes.
+- `course/portfolio/`: progress, experiment logs, implementation notes, and resume draft.
+
+Start here:
+
+- [Course syllabus](course/00_syllabus.md)
+- [Final review and capstone](course/28_final_review_and_capstone.md)
+- [Course directory README](course/README.md)
+
+Notes:
+
+- The original MiniMind source remains authored by the MiniMind project.
+- This course is an added learning layer for reading, tracing, and re-implementing selected concepts from the source.
+- Model weights such as `minimind-3/`, `*.safetensors`, and `*.pth` are not committed here.