Add note on compile_commands.json to readme

README.md

@@ -28,6 +28,15 @@ but you can find them in `flake.nix`.
 
 You can use `nix develop` or [`direnv`](https://direnv.net/).
 
+To create a `compile_commands.json` for `ccls` (or similar),
+create a `build` directory inside the exercise directory,
+and from that, execute:
+
+```bash
+cmake .. -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=YES
+ln -sf build/compile_commands.json ..
+```
+
 ## Licence
 
 [GPL-3.0-or-later](https://spdx.org/licenses/GPL-3.0-or-later.html).