Document VIE's schema import format

[Sebastian Lorenz]

Using http://schema.rdfs.org/all.json as an example is a bit painful because it is so verbose. Something like:

{
  "datatypes": {
    "isFamilyFriendly": {
      "comment": "Indicates whether this content is family friendly.", 
      "comment_plain": "Indicates whether this content is family friendly.", 
      "domains": [
        "CreativeWork"
      ], 
      "id": "isFamilyFriendly", 
      "label": "Is Family Friendly", 
      "ranges": [
        "Boolean"
      ]
    }
  }, 
  "types": {
    "AboutPage": {
      "ancestors": [
        "Thing", 
        "WebPage"
      ], 
      "comment": "Web page type: About page.", 
      "comment_plain": "Web page type: About page.", 
      "id": "AboutPage", 
      "label": "About Page", 
      "properties": [
        "isFamilyFriendly"
      ], 
      "specific_properties": [], 
      "subtypes": [], 
      "supertypes": [
        "WebPage"
      ], 
      "url": "http://schema.org/AboutPage"
    }, 
  }, 
  "valid": "2012-11-03"
}

This would help also with openpsa/createphp#24