Add nix related section/debug FAQ in contributor's guide

[harendra-kumar]

• Explain basic use of default.nix for different use cases

what to do if you are facing a problem like:

[1] unknown package: bench-show (dependency of bench-chart)

Use ghc-pkg list to see if the pkg is visible to ghc.

• If not, is the package listed as a dependency in any of the cabal files?
• If yes, is the cabal file listed in default.nix being used
• spelling mistakes in nix-shell arguments are silently ignored