Included new field for /coins/{id} endpoint

We've included a new field "whitepaper" under "links" section for /coins/{id} endpoint

Example of responses:

{
  "id": "bitcoin",
  ......
  "links": {
    "homepage": [],
    "whitepaper": "https://bitcoin.org/bitcoin.pdf", 👈
    "blockchain_site": [],
    "official_forum_url": [],
    "chat_url": [],
    "announcement_url": [],
    "twitter_screen_name": "bitcoin",
    "facebook_username": "bitcoins",
    "bitcointalk_thread_identifier": null,
    "telegram_channel_identifier": "",
    "subreddit_url": "https://www.reddit.com/r/Bitcoin/",
    "repos_url": {}
  },
  .......
}