docs(test): suggest a workaround for the slow admin test suite

We're unlikely to invest the time in optimizing the admin test suite's
container as we've done for SecureDrop.  Barring that optimization,
others may find that this hint saves them some time.

admin/Makefile

@@ -2,6 +2,7 @@ DEFAULT_GOAL: help
 
 .PHONY: test
 test:  ## Run tox
+	@echo "NB.  This can be VERY slow.  If you find yourself running this test suite multiple times, you may prefer to \"docker run -it securedrop-admin bash\", install the editor of your choice into the container and edit there, and run \"tox\" directly as you work."
 	bin/dev-shell tox
 
 .PHONY: update-pip-requirements