Make a note to update Dependabot config

If `requirements.in` is changed, the list of dependencies in the
Dependabot config file should be updated too.

Author: mhucka

src/py/requirements.in

@@ -1,4 +1,7 @@
+# Note: if you add or subtract from the list below, also update the "allow"
+# list in ../../.github/dependabot.yaml.
+
 stim
 pytest
 sinter
-pybind11-stubgen
+pybind11-stubgen