Change autofocus="true" to autofocus="" per HTML5 spec

http://www.w3.org/html/wg/drafts/html/master/infrastructure.html#boolean-attribute

The presence of a boolean attribute on an element represents the true
value, and the absence of the attribute represents the false value.

If the attribute is present, its value must either be the empty string
or a value that is an ASCII case-insensitive match for the attribute's
canonical name, with no leading or trailing whitespace.

I understand this is a little anal since autofocus="true" seems to work just fine. Technically, I believe you can also use autofocus="autofocus" but I decided to keep it simple in the `README`. Of course, feel free to elaborate/reword if you want to.

I only noticed this because I was trying to figure out why it didn't work in Firefox, turns out that while Firefox supports autofocus, it doesn't seem to like when you inject it, boo Firefox, so obviously this change doesn't help that situation any.