feat: update README and add documentation for installation, configuration, usage, and plugin development

docs/usage.md

@@ -0,0 +1,50 @@
+# Usage
+
+## Running
+
+```bash
+k-launcher
+```
+
+## Keybinds
+
+| Key | Action |
+|-----|--------|
+| Type | Filter results |
+| `↑` / `↓` | Navigate list |
+| `Enter` | Launch selected result |
+| `Escape` | Close launcher |
+
+## Built-in Plugins
+
+### Apps
+
+Type any app name to search installed applications. An empty query shows your most frequently launched apps (frecency-ranked top results).
+
+### Calc
+
+Type a math expression — the result appears instantly and is copied to clipboard on `Enter`.
+
+```
+2^10 + 5       → 1029
+sqrt(144)      → 12
+sin(pi / 2)   → 1
+```
+
+### Shell Command
+
+Prefix your input with `>` to run a shell command in a terminal:
+
+```
+> echo hello
+> htop
+```
+
+### Files
+
+Start your query with `/` or `~/` to browse the filesystem:
+
+```
+/etc/hosts
+~/Documents/report.pdf
+```