Docs: Fix typos

Close h5bp/html5-boilerplate#1600
Update content from the `dist` directory [skip ci]
Docs: Add information about the `robots.txt` file

Ref: h5bp/html5-boilerplate#1554
Close: h5bp/html5-boilerplate#1598
Docs: Provide link to Google's docs on robots.txt

Ref: h5bp/html5-boilerplate#1554
Remove the `cross-domain` phrase from `README.md`

The part about `Cross-domain Ajax and Flash` from the `README.md` file
isn't accurate, as by default:

 * the `crossdomain.xml` file doesn't grant a web client — such as Adobe
   Flash Player, Adobe Reader, etc. — permission to handle data across
   multiple domains

 * the Apache server configs, do not allow cross-origin access to all
   resources, unless the user enables that behavior
Futher optimize `tile.png` and `tile-wide.png`

 * `tile.png`:

    before:        13743 B
    after:          3495 B
    ──────────────────────
    reduction:     10248 B [74.56%]

 * `tile-wide.png`

    before:         6774 B
    after:          1864 B
    ──────────────────────
    reduction:      4910 B [72.48%]

Close h5bp/html5-boilerplate#1604.
Update `apple-touch-icon-precomposed.png` and docs

 * Update the documentation regarding the Apple touch icons, by
   reflecting the recent changes brought by the release of iOS 8 and the
   new iPhone 6 and 6 Plus.

   Note: The changes do not follow all Apple's iOS Human Interface
   Guidelines¹, as in practice, the images used by the different devices
   are different² from the ones specified in the Apple documentation.

 * Update the `apple-touch-icon-precomposed.png` to be `180×180px` (this
   is now the largest image size used by any Apple device running iOS).

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

¹ https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/IconMatrix.html
² h5bp/html5-boilerplate#1599

Close h5bp/html5-boilerplate#1599.