doc: readme: describe cognitect test-runner opts

Also add a note on why we have to 2 REPL launchers.

Author: lread

README.md

@@ -121,6 +121,11 @@ To run default tests only:
 ```
 bb test
 ```
+You can also use [cognitect test-runner](https://github.com/cognitect-labs/test-runner?tab=readme-ov-file#invoke-with-clojure--m-clojuremain) options, for example, to run a single test:
+```
+bb test --var babashka.fs-test/walk-test
+```
+
 To fire up a REPL when working on these tests, run `bb dev`.
 
 ### Scratch CWD Tests
@@ -129,10 +134,18 @@ To run these tests only:
 ```
 bb test-cwd
 ```
+As above, cognitect test-runner options work:
+```
+bb test-cwd --var babashka.fs-cwd-test/es-copy-test
+```
+
 The scratch current working directory is `target/test-cwd`.
 To fire up a REPL for these tests, run `bb dev-cwd`.
 Note that the `.nrepl-port` file will be generated under `target/test-cwd/`, so you'll have to type the REPL port in manually when connecting.
 
+> [!NOTE]
+> We currently have 2 REPL launcher tasks, `dev` and `dev-cwd`, because they each rely on a different current working directory.
+
 ### API Docs
 This project generates API docs with quickdoc, to regenerate `API.md`:
 ```