Small changes wrt to documentation and best practices

[pmerkleplant]

• Fixes chronicle-std dependency to version 1, enabling reproducible builds
• Adds installation and contributing sections to readme
• Adds a config file to run slither against repo easily
• Use chronicle-std's IChronicle interface instead of local version
• Adds optimized - wrt to function selector - poke() function
• Set configuration (spread and stalenessThreshold) in constructor via functions to emit appropriate events
  • This allows offchain indexers to reconstruct aggor's state just via event monitoring
• Adds a decimals()(uint8) function to increase Chainlink compatibility. Decimals ist hardcoded to 18.
• Adjusts latestRoundData() function to return a roundId and answeredInRound of 1.
  • These values should not be zero as tutorials encourage to interpret such values as failures
• Adjust setSpread() function to be idempotent and fail on invalid input
• Some documentation updates