Update README

[nochiel]

The README has sections that are out of date with respect to the available functionality. The sections that need to be updated are:

• Usage instructions
  • Spotbit's API is now self-documenting. This section isn't needed.
• https://github.com/BlockchainCommons/spotbit#config-options
• https://github.com/BlockchainCommons/spotbit#exchanges-used-for-averaging
• https://github.com/BlockchainCommons/spotbit#dependencies