nix readme update

Author: peterbecich

README.md

@@ -20,9 +20,9 @@ uses the [icu4c](http://icu-project.org/apiref/icu4c/) library to build.
 If you have the Nix package manager installed, the easiest way to obtain
 `hackage-server`'s dependencies is using the Nix shell:
 
-    nix-shell
+    nix-shell --pure
 
-Note: `libbrotli-dev` has to be installed manually.
+Nix dependencies are managed by [Niv](https://github.com/nmattia/niv). Pull the latest version of the current [channel](https://nixos.wiki/wiki/Nix_channels) with `niv update`. Upgrade to a newer channel by editing `nix/sources.json`, and then `niv update`.
 
 ### Manually
 

nix/sources.json

@@ -5,10 +5,10 @@
         "homepage": "https://github.com/nmattia/niv",
         "owner": "nmattia",
         "repo": "niv",
-        "rev": "9cb7ef336bb71fd1ca84fc7f2dff15ef4b033f2a",
-        "sha256": "1ajyqr8zka1zlb25jx1v4xys3zqmdy3prbm1vxlid6ah27a8qnzh",
+        "rev": "5912c378c2f87e911f1bc7236202f69581406c9d",
+        "sha256": "1jhs562k3pk0xgwyf0m1h2bck4rivv9f3aznz8dslgby9g5mdnda",
         "type": "tarball",
-        "url": "https://github.com/nmattia/niv/archive/9cb7ef336bb71fd1ca84fc7f2dff15ef4b033f2a.tar.gz",
+        "url": "https://github.com/nmattia/niv/archive/5912c378c2f87e911f1bc7236202f69581406c9d.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "nixpkgs": {
@@ -17,10 +17,10 @@
         "homepage": "",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "bce8fb8760db683bd69e42633fd9a610b52e2afc",
-        "sha256": "0039n93lhnqcnw1mrl4f2g01r4qs91g8wl9dgbx0qmvrl6xzaqwf",
+        "rev": "dfa6cc88931d021ada6cd51f05095afa95be83a7",
+        "sha256": "1f8j18zdpg1gvz4a9mgzyvwxdj8fh3y5xlkrnn5zynmswdmpj2gq",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/bce8fb8760db683bd69e42633fd9a610b52e2afc.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/dfa6cc88931d021ada6cd51f05095afa95be83a7.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }