Docs: add new files to `usage.md`

Updated `usage.md` to include 4 new files included with the project:

* `.editorconfig`
* `tile-wide.png`, `tile.png` and `browserconfig.xml`

Ref https://github.com/h5bp/html5-boilerplate/commit/80530d6b4639980f3ab0858123edbd730745fcde
    https://github.com/h5bp/html5-boilerplate/commit/a99d8563ddc12cbf941057ee1b8b614fe2c9378c
Close #1647