Add Getting Started

[nettle]

We should add quick Getting Started section (probably instead of Prerequisites, or make Prerequisites as a part of Getting Started) to make sure that environment can be set correctly.

Scope

1. List supported environments (host platforms), e.g.:
   • Ubuntu (versions?)
   • RHEL (versions?)
2. Reveal all environment requirements and define prerequisites (Update Prerequisites #29)
   • CodeChecker, Bazel, Python, Clang (versions?)
   • Disable/remove ccache?
   • Rename clang-extdef-mapping?
   • What else?
3. Develop simple shell commands for supported environments
4. Test environment setup
5. Update documentation
   • README
   • CONTRIBUTING (if needed)

DoD (Definition of Done)

Documentation is updated and tested