Add secrets.yaml.example so required keys are discoverable

[malaiwah]

The YAML uses !secret references for five values:

• wifi_ssid
• wifi_password
• wifi_fallback_password
• ota_password
• home_assistant_key

Currently undocumented anywhere — new users have to read the YAML to figure out what to put in their secrets.yaml. ESPHome conventionally ships a secrets.yaml.example.

PR follows.