Add discussion of patching functionality to indicator manual

[Example patching module](https://github.com/cmu-delphi/covidcast-indicators/blob/main/doctor_visits/delphi_doctor_visits/patch.py).

[Indicator manual](https://github.com/cmu-delphi/covidcast-indicators/blob/main/_template_python/INDICATOR_DEV_GUIDE.md).