DOC: pytest --nightly (#267)

Author: twoertwein

docs/tests.md

@@ -11,6 +11,7 @@ Here are the most important options. Fore more details, please use `poe --help`.
   - Run only pytest: `poe pytest`
   - Run only pre-commit: `poe style`
 - Run tests against the installed stubs (this will install and uninstall the stubs): `poe test_dist`
+- Optional: run pytest against pandas nightly (this might fail): `poe pytest --nightly`
 - Optional: Run stubtest to compare the installed pandas-stubs against pandas (this will fail): `poe stubtest`. If you have created an allowlist to ignore certain errors: `poe stubtest path_to_the_allow_list`
 
 These tests originally came from https://github.com/VirtusLab/pandas-stubs.