Adds global filters docs for #571

reviewer: @ebidel 
This is a little vague, wondering if we should include as a live example so people could see the exact file contents we're talking about.

"define it in a script" could mean a separate JS file or an inline script tag. Below, you show using an HTML import, so the connection isn't completely clear. I know the HTML import could include either inline script or a link to an external script, but is either one preferable to the other?

Probably overkill, but a demo tab showing "uppercase-filter.js", "uppercase-filter.html", and an index.html with an `<input>` and a `{{value | uppercase}}` would show all the bits working together.