Add more readme for integration tests (#4515)

cli/integration_tests/README.md

@@ -16,13 +16,29 @@ Setup
   $ . ${TESTDIR}/setup_monorepo.sh $(pwd)
 ```
 
-- `setup.sh` sets a `TURBO` environment variable that points to the locally built binary
+- `setup.sh` sets a `TURBO` environment variable that points to the locally built binary.
 - `setup_monorepo.sh` uses one of the test repos in the `_fixtures` directory to exercise
   the `TURBO` binary against.
 
 ### Fixtures
 
 For the most part, use the `basic_monorepo`, or `single_package` fixtures to test against.
+By default the script will use `basic_monorepo`, but you can specify the fixture with a second
+argument:
+
+```bash
+  $ . ${TESTDIR}/setup_monorepo.sh $(pwd) single_package
+```
+
+You can also pass a second argument to change the packageManager of a fixture:
+
+```bash
+  $ . ${TESTDIR}/setup_monorepo.sh $(pwd) basic_monorepo "yarn@1.22.17"
+```
+
+Note that if you want to customize the package mangaer, you'll have to specify the fixture name
+also since the script just uses positional arguments.
+
 You can add custom fixture monorepos as an escape hatch or if you truly need a custom monorepo.
 
 #### Custom turbo.json