docs: add README and update source layout

Add a brief project README and list it in the AGENTS.md source
layout block.

Co-authored-by: Qwen 3.8 27B <qwen3.8_27b-q3_k_xl@jelly.nrv.nu>

AGENTS.md

@@ -95,6 +95,7 @@ releases of the course material.
 ```text
 cppcourse/
   AGENTS.md         # This file
+  README.md         # Project docs
   .oxfmtrc.json     # oxfmt config
   .agents/          # Agent skills (git workflow)
 ```

README.md

@@ -0,0 +1,8 @@
+# cppcourse
+
+Course material for a C++ course: lecture notes, exercises, and runnable code
+examples in modern C++ (C++23), built and run on Linux.
+
+The repository is in a startup phase; content will be added as the course
+develops. Coding conventions and agent instructions live in
+[AGENTS.md](AGENTS.md).