Prompted to create this for present and future Jekyll projects after seeing a link on adactio.com to this Manifest Generator which then led me to find a good reason (article), Installable Web Apps, for such thing.

_config.yml

# Manifest settings (Used to make manifest.json)
# Edit manifest.json directly concerning icon information
# A sample icon is in the /img directory and manifest.json references that icon.
manifest_name: Web App Full Name
manifest_short_name: Short Name # Appears in "Add to home screen" dialogues and may have other importance also
manifest_lang: en
manifest_background_color: white
manifest_short_name:
manifest_display: standalone

manifest.json

---
permalink: manifest.json
---

{
  "lang": "",
  "background_color": "#",
  "name": "",
  "short_name": "",
  "display": "",
  "icons": [
    {
      "src": "img/144X144-icon.png",
      "sizes": "144x144",
      "type": "image/png"
    }
  ]
}

See Github repository.