[DEVOPS-268] iohk cache (#1830)
[DEVOPS-268] iohk cache (#1830)

* [DEVOPS-268] README.md:  document building Cardano SL using Nix

* [DEVOPS-268]  readme | building:  deprecate build instructions and refer to cardanodocs.com, instead

input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype