[clangd] add cmake files and guide for clangd usage

README.md

@@ -1,3 +1,13 @@
 # practices
+Repository with code from practice sessions
 
-Repository with code from practice sessions
+# Clangd
+First, build the project to generate `compile_commands.json`:
+```bash
+cmake -B build
+```
+Then tell clangd where to find it by adding to `.clangd`:
+```yaml
+CompileFlags:
+  CompilationDatabase: build
+```