Docs: Update file tree in `usage.md`

Added to the file tree + with a little of info about each:

* `.editorconfig`, added in https://github.com/h5bp/html5-boilerplate/commit/a99d8563ddc12cbf941057ee1b8b614fe2c9378c.

* `tile-wide.png`, `tile.png` and `browserconfig.xml`: added in https://github.com/h5bp/html5boilerplate.com/commit/55709c9ce642ed7509ae3275cad2089fc45219d5.

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If this gets merged, somebody may tick of the `usage.md` item in https://github.com/h5bp/html5-boilerplate/issues/1554. Nothing more to be updated in there (I think).