docs: document configuration

README.md

@@ -1,5 +1,70 @@
 # infra-rebuild
 
+infra-rebuild mirrors nixos-rebuild closely in its command-line interface, but focuses on making infrastructure deployment more convenient. It is built for infrastructure defined in a flake, where each hosts configuration is represented by a different `nixosConfiguration` in the flakes output.
+
+For building the configuration of e.g. two hosts called `web-01` and `git` one can simply run:
+
+```
+infra-rebuild build web-01 git
+```
+
+For deploying the hosts - using the switch operation - it's as simple as:
+
+```
+infra-rebuild switch web-01 git
+```
+
+Here infra-rebuild will simply run `nixos-rebuild switch` with the `target-host` option set for each host.
+Because infra-rebuild tries to be usable without any configuration, by default it queries the hosts FQDN from its `nixosConfiguration` and uses that for the target host.
+However to override aspects of the target host for specific or all hosts, infra-rebuild also accepts a `deployment_configuration.json` as configuration.
+
+## Configuration
+
+infra-rebuild accepts optional configuration in a `deployment_configuration.json`. \
+The following keys are available to be set for configuring various aspects of deployment for specific or all hosts:
+
+- `default.targetPort`: A default port to use for connecting to all host.
+- `default.targetUser`: A default user to use for connecting to all host.
+- `hosts.<host>.targetPort`: The port to use for connecting to `<host>`.
+- `hosts.<host>.targetUser`: The user to use for connecting to `<host>`.
+- `hosts.<host>.targetHostname`: The hostname to use for connecting to `<host>`.
+
+### Example
+
+An example `deployment_configuration.json` might look like this then:
+
+```json
+{
+  "default": {
+    "targetPort": 2222,
+    "targetUser": "deploy"
+  },
+  "hosts": {
+    "web-01": {
+      "targetUser": "web-deploy",
+      "targetHostname": "web-01-intern.infra.example",
+      "targetPort": 22
+    }
+  },
+  "git": {
+    "targetUser": "git-deploy"
+  }
+}
+```
+
+### Order of Precedence
+
+The order of precedence from least to greatest for `targetPort` and `targetUser` is the following:
+
+1. system default
+2. `default.targetPort/User`
+3. `hosts.<host>.targetPort/User`
+
+The order of precedence from least to greatest for `targetHostname` is the following:
+
+1. FQDN from hosts nixosConfiguration
+2. `hosts.<host>.targetHostname`
+
 ## License
 
 This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.