Support per-node Nixpkgs overrides and local deployment

Also renamed the `network` key to `meta`.

README.md

@@ -17,13 +17,18 @@ Here is a sample `hive.nix` with two nodes, with some common configurations appl
 
 ```nix
 {
-  network = {
+  meta = {
     # Override to pin the Nixpkgs version (recommended). This option
     # accepts one of the following:
     # - A path to a Nixpkgs checkout
     # - The Nixpkgs lambda (e.g., import <nixpkgs>)
     # - An initialized Nixpkgs attribute set
     nixpkgs = <nixpkgs>;
+
+    # You can also override Nixpkgs by node!
+    nodeNixpkgs = {
+      node-b = ./another-nixos-checkout;
+    };
   };
 
   defaults = { pkgs, ... }: {
@@ -83,6 +88,59 @@ Then you can evaluate with:
 colmena introspect your-lambda.nix
 ```
 
+## `colmena apply-local`
+
+For some machines, you may still want to stick with the manual `nixos-rebuild`-type of workflow.
+Colmena allows you to build and activate configurations on the host running Colmena itself, provided that:
+
+1. The node must be running NixOS.
+1. The node must have `deployment.allowLocalDeployment` set to `true`.
+1. The node's _attribute name_ must match the hostname of the machine.
+
+If you invoke `apply-local` with `--sudo`, Colmena will attempt to elevate privileges with `sudo` if it's not run as root.
+You may also find it helpful to set `deployment.targetHost` to `null` if you don't intend to deploy to the host via SSH.
+
+As an example, the following `hive.nix` includes a node (`laptop`) that is meant to be only deployed with `apply-local`:
+
+```nix
+{
+  meta = {
+    nixpkgs = ./deps/nixpkgs-stable;
+
+    # I'd like to use the unstable version of Nixpkgs on
+    # my desktop machines.
+    nodeNixpkgs = {
+      laptop = ./deps/nixpkgs-unstable;
+    };
+  };
+
+  # This attribute name must match the output of `hostname` on your machine
+  laptop = { name, nodes, ... }: {
+    networking.hostName = "laptop";
+
+    deployment = {
+      # Allow local deployment with `colmena apply-local`
+      allowLocalDeployment = true;
+
+      # Disable SSH deployment. This node will be skipped in a
+      # normal`colmena apply`.
+      targetHost = null;
+    };
+
+    # Rest of configuration...
+  };
+
+  server-a = { pkgs, ... }: {
+    # This node will use the default Nixpkgs checkout specified
+    # in `meta.nixpkgs`.
+
+    # Rest of configuration...
+  };
+}
+```
+
+On `laptop`, run `colmena apply-local --sudo` to activate the configuration.
+
 ## Current limitations
 
 - It's required to use SSH keys to log into the remote hosts, and interactive authentication will not work.