Set Jekyll encoding and tweak Windows notes

Via https://github.com/juthilo/run-jekyll-on-windows/issues/1 : Since v1.3.0, there's a new `encoding` option in Jekyll. If set to `UTF-8`, Windows users no longer need to manually change their command prompt's character encoding for a site. (I tested this locally.)

This adds `encoding: UTF-8` to our `_config.yml` and tweaks the "Windows users:" notes in the README.

@mdo @cvrebert Thoughts? Would love to get this into 3.1, given it's a relatively small change.