Remove `gulp-gh-pages` and document alternative approach instead

For some reason I got weird errors out of `gulp-gh-pages` when trying to
deploy (first deploy was fine). Likely it expects to work from master
branch or something.

Anyhow, looks like there's an alternative way to achieve the same result.
The approach could be integrated to the gulp configuration even so then
you could do `gulp deploy` or a githook performing the step could be set
up.