Document how to use watchexec to re-run recipes when files change (#2036)

README.md

@@ -3047,12 +3047,19 @@ can also be used to make a `just` binary print its changelog.
 Miscellanea
 -----------
 
-### Companion Tools
+### Re-running recipes when files change
 
-Tools that pair nicely with `just` include:
+[`watchexec`](https://github.com/mattgreen/watchexec) can re-run any command
+when files change.
 
-- [`watchexec`](https://github.com/mattgreen/watchexec) — a simple tool that
+To re-run the recipe `foo` when any file changes:
-  watches a path and runs a command whenever it detects modifications.
+
+```sh
+watchexec just foo
+```
+
+See `watchexec --help` for more info, including how to specify which files
+should be watched for changes.
 
 ### Shell Alias