[w3c/manifest] Add (optional) "category" member (#569)

Many app stores including [Apple](https://developer.apple.com/app-store/categories/), [Google](https://support.google.com/googleplay/android-developer/answer/113475?hl=en), and [Microsoft](https://docs.microsoft.com/en-us/windows/uwp/publish/category-and-subcategory-table) ask developers to choose a "category" for their app.  This would be a good (optional) field to enable web app developers to suggest which category their app should be listed in.

```json
  "category": "sports",
```

**FAQ:**
**Q: These stores (and others) don't have the same categories.  How would you align?**
A: Actually, they have many of the same categories: Business, Education, Entertainment, Health & Fitness, Lifestyle, Medical, Productivity, Shopping, and Sports
A: And may categories that are very similar:
* Books, Books & Reference, Books + reference
* Food & Drink, Food + dining
* Magazines & Newspapers, News & Magazines, 
* Music, Music & Audio
* Navigation, Maps & Navigation, Navigation + maps 

**Q: What about sub-categories?**
A: Similar to the old "font" tag, web app developers could list desired categories in descending specificity.
For example: `"category": "Hotels, Travel, Lifestyle"`

**Q: What about non-English strings?**
A: Just as other enumerations in the Manifest use English strings, these categories would use English strings. 
However, there would be nothing preventing a developer from listing non-English strings. 
For example: `"category": "Libros, Books, Reference"`

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/569