add princeton 2026 quickstart doc (#54)

Author: msaroufim

docs/princeton2026.md

@@ -0,0 +1,29 @@
+# Princeton 2026 Quick Start
+
+Use `A100` and this leaderboard: `princeton_cross_entropy`.
+
+```bash
+# 1. Install the CLI
+curl -fsSL https://raw.githubusercontent.com/gpu-mode/popcorn-cli/main/install.sh | bash
+
+# 2. Register once with GitHub
+popcorn register github
+
+# 3. Join the closed leaderboard with your invite code
+popcorn join <YOUR_INVITE_CODE>
+
+# 4. Get the starter file
+wget https://raw.githubusercontent.com/gpu-mode/reference-kernels/main/problems/princeton/cross_entropy_py/submission.py
+
+# 5. Run a correctness check
+popcorn submit --leaderboard princeton_cross_entropy --gpu A100 --mode test submission.py
+
+# 6. Submit an official ranked run
+popcorn submit --leaderboard princeton_cross_entropy --gpu A100 --mode leaderboard submission.py
+```
+
+Notes:
+
+- `test` checks correctness only.
+- `leaderboard` is the official ranked submission.
+- If registration gets stuck, run `popcorn reregister github`.