Add list of APIs used by Preact

As suggested in developit/preact#585. Here's the list I came up with:

Browser APIs Preact Uses

• It will use Promise only if available, falling back to setTimeout(0)
• DOM APIs used are very minimal DOM core 1 / 2:
  • document.createElement() and document.createTextNode()
  • Text.prototype.nodeValue
  • Text.prototype.splitText - simply needs to exist (used as fast "is text node" check)
  • From Element.prototype:
    • .attributes NodeList
    • .setAttribute(), .getAttribute(), .removeAttribute()
    • .appendChild(), .insertBefore(), .replaceChild()

If you use SVG, createElementNS() and (get|set|remove)AttributeNS() are used.