billing: Add mypy daemon warning to stripe stubs file.

stubs/stripe/__init__.pyi

@@ -1,3 +1,9 @@
+# NOTE: When editing stubs, we recommend using `tools/run-mypy --no-daemon`,
+# because the mypy daemon's caching system has some bugs around editing stubs
+#
+# See https://zulip.readthedocs.io/en/latest/testing/mypy.html#mypy-stubs-for-third-party-modules
+# for notes on how we manage mypy stubs.
+
 import stripe.error as error
 import stripe.util as util
 import stripe.api_requestor as api_requestor