Look into adding a web application manifest file

## General information

From: https://w3c.github.io/manifest/#h2_abstract

> JSON-based manifest, which provides developers with a centralized place to put metadata associated with a web application. This includes, but is not limited to, the web application's name, links to icons, as well as the preferred URL to open when a user launches the web application. The manifest also allows developers to declare a default orientation for their web application, as well as providing the ability to set the display mode for the application (e.g., in fullscreen). .

From: http://blog.chromium.org/2014/10/chrome-39-beta-js-generators-animation.html

> Previously when developers wanted to allow their web applications to be added to the home screen from Chrome for Android, they had to use a variety of `<meta>` and `<link>` tags to trigger this behavior and deliver relevant resources such as icons. Having this embedded in every page was not only repetitive, but was a waste of bandwidth and put extra bits on the critical path.
> ... 
> Developers seeking to enable "add to homescreen" can define a title, landing page, default orientation, and different icons depending on size and screen density. 

## Links

Specification: 
* https://w3c.github.io/manifest/

Other useful links:
* http://html5doctor.com/web-manifest-specification/

## Browser support
 
 - [x] [Chrome 39+](http://blog.chromium.org/2014/10/chrome-39-beta-js-generators-animation.html)
 - [ ] Firefox - not yet, but [underway](https://bugzilla.mozilla.org/show_bug.cgi?id=997779)!
 - [ ]  Internet Explorer - not yet, but [it's under consideration](https://status.modern.ie/webapplicationmanifest)!<br>
   (@jacobrossi it would be really great if Internet Explorer would use the web application manifest file instead of the very specific [`/browserconfig.xml`](https://github.com/h5bp/html5-boilerplate/pull/1481))
 - [x]  Opera 26+ (?)
 - [ ]  Safari - not yet

---

## Required changes 

 - [ ] Add a generic web application manifest file
    - [ ] Decide on the name (e.g.: `manifest.json`)
    - [ ] Decide what members to provide by default and with what values<br>
        (e.g.: obviously [`"icons"`](https://w3c.github.io/manifest/#h3_icons-member) needs to be provided with multiple icon objects, but [`"name"`](https://w3c.github.io/manifest/#name-member) does not as its value defaults to the value of the `<title>` element and that is a better default for out case)
 - [ ] Add a `<link>` in `index.html`(e.g.:  `<link rel="manifest" href="manifest.json">`)
 - [ ] Update documentation

The server configs are [already covered](https://github.com/h5bp/server-configs-apache/issues/29)!

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Note: For now the scope of this issue is mainly to start the discussion and to track how browser support evolves.