Add documentation for library consumers

[patrislav]

• Improve and point to https://pkg.go.dev/ (once open-sourced)
• Document AWS-specific info and current limitations:
  • EC2: only on-demand Linux instances with no pre-installed software are supported (e.g. no spot, reserved, Windows, SQL Server, etc.)
  • EC2: only the Compute and Storage costs are calculated (e.g. no GPU, monitoring, etc.)
  • document all the defaults and assumptions that are used in the calculations (e.g. for EC2: 730 hours in a month, for EBS: the "gp2" type, 8 GB storage, etc.) - this should include links to the appropriate provider docs
• README and "how to contribute" guide
• Add "how does it work" section, explaining the way the library interacts with the DB and the way cost is estimated

[patrislav]

Reopening, not all the points were finished yet.