docs(readme): Add info about using the Cachix binary cache

Author: PetarKirov

README.md

@@ -27,25 +27,44 @@ nix shell github:metacraft-labs/nix-blockchain-development#solana
 
   ```nix
   {
+    # <after>:
+    # Opt into `nix-blockchain-development`'s substituter (binary cache).
+    # `nixConfig` settings are not transitive so every user of a flake with a
+    # custom binary cache must manually include its `nixConfig` settings for
+    # substituters and trusted public keys:
+    nixConfig = {
+      extra-substituters = "https://nix-blockchain-development.cachix.org";
+      extra-trusted-public-keys = "nix-blockchain-development.cachix.org-1:Ekei3RuW3Se+P/UIo6Q/oAgor/fVhFuuuX5jR8K/cdg=";
+    };
+
     inputs = {
-      nixpkgs.url = "github:NixOS/nixpkgs/nixos-22.05";
-      flake-utils.url = github:numtide/flake-utils;
-      mcl-blockchain.url = "github:metacraft-labs/nix-blockchain-development"; # <new>
-      mcl-blockchain.inputs.nixpkgs.follows = "nixpkgs";                       # <new>
-      mcl-blockchain.inputs.flake-utils.follows = "flake-utils";               # <new>
+      # <before>:
+      # nixpkgs.url = "github:NixOS/nixpkgs/nixos-22.05";
+      # flake-utils.url = github:numtide/flake-utils;
+
+      # <after>:
+      # To ensure all packages from mcl-blockchain will be fetched from its
+      # binary cache we need to ensure that we use exact same commit hash of the
+      # inputs below. If we didn't, we may either:
+      # * end up with multiple copies of the same package from nixpkgs
+      # * be unable to use the binary cache, since the packages there where
+      #   using different versions of their dependencies from nixpkgs
+      mcl-blockchain.url = "github:metacraft-labs/nix-blockchain-development";
+      nixpkgs.follows = "mcl-blockchain/nixpkgs";
+      flake-utils.follows = "mcl-blockchain/flake-utils";
     };
 
     outputs = {
       self,
       nixpkgs,
       flake-utils,
-      mcl-blockchain, # <new>
+      mcl-blockchain, # <after>
     }:
       flake-utils.lib.simpleFlake {
         inherit self nixpkgs;
         name = "solana-hello-world";
         shell = ./shell.nix;
-        preOverlays = [mcl-blockchain.overlays.default]; # <new>
+        preOverlays = [mcl-blockchain.overlays.default]; # <after>
       };
   }
   ```
@@ -57,7 +76,7 @@ nix shell github:metacraft-labs/nix-blockchain-development#solana
   with pkgs;
     mkShell {
       packages = [
-        metacraft-labs.solana # <new>
+        metacraft-labs.solana # <after>
       ];
     }
   ```