📚 Add installation instructions

Author: davep

README.md

@@ -4,6 +4,16 @@ A quick spur-of-the-moment hack that will likely grow into something a wee
 bit more useful. For now though, it's a simple/pragmatic tool for quickly
 testing [Textual queries](https://textual.textualize.io/guide/queries/).
 
+## Installation
+
+Install either with `pip` or (ideally) `pipx`:
+
+```sh
+$ pipx install textual-query-sandbox
+```
+
+Then run `tqs`. That's it!
+
 ![Textual Query Sandbox](./tqs.png)
 
 [//]: # (README.md ends here)