txscript: scriptPubKey and scriptSig

[jholdstock]

Explain the difference between, and purpose of scriptPubKey and scriptSig.

Some good reference for this:

• https://link.springer.com/chapter/10.1007/978-1-4842-5522-3_7
• https://www.mycryptopedia.com/bitcoin-script-explained/

Not including this in #86 because that PR is growing large already