Add coverage of DLTs

[dbosk]

Potential material:

• https://elibrary.worldbank.org/doi/abs/10.1596/29053 or http://documents.worldbank.org/curated/en/177911513714062215/pdf/122140-WP-PUBLIC-Distributed-Ledger-Technology-and-Blockchain-Fintech-Notes.pdf
• There is some interesting material available at
  http://bitcoinbook.cs.princeton.edu/
• https://csrc.nist.gov/publications/detail/nistir/8202/draft
• IEEE S&P Special Issue on Blockchain https://ieeexplore.ieee.org/xpl/tocresult.jsp?isnumber=8425603

[dbosk]

I added videos from Princeton's Bitcoin Book (to ScalableLearning) and some sections from NIST's blockchain overview (to compsec study guide).