Suggestion for installation guide

[hutchwilco]

Suggest the secrets.yaml instructions clarify that the entries:

pv_inverter_api_key:
pv_inverter_ota_password:
pv_inverter_fallback_password:

should carry the explicit explanation that these are to be generated by the user ( ie defined here first) rather than retrieved from the inverter or some other place, as the names tend to imply.