Update README for `plutus-contract` library

[vyastrebovvareger]

Describe the feature you'd like

Could you please provide the correct steps to how to build the examples?
Currently as far as I can see it's out of date because

1. plutus-contract folder does not have any nix config file => 1. cd plutus-contract 2. nix-shell is not working
2. in cabal there is no contract-exe-guessing-game entry point.

Describe alternatives you've considered

No response

[ak3n]

Thanks! I have update the README. It was very outdated.

Please take a look at plutus-use-cases for the examples.