Update documentation in README.md.

README.md

@@ -24,8 +24,62 @@ Under development.
 
 Provide a simple Config class that can be injected where needed to provide configuration values in non-command classes, and make configuration settings a no-op for command classes by automatically initializing the Input object from configuration as needed.
 
+## Configuration File Usage
+
+Configuration files are simple hierarchical yaml files.
+
+### Providing Command Options
+
+Command options are defined by creating an entry for each command name under the `command:` section of the configuration file. The options for the command should be defined within an `options:` section. For example, to set a configuration value `red` for the option `--color` in the `example` command:
+```
+command:
+  example:
+    options:
+      color: red
+```
+If a command name contains a `:`, then each section of the command name defines another level of heirarchy in the command option configuration. For example, to set a configuration value `George` for the option `--name` of the command `my:foo`:
+```
+command:
+  my:
+    foo:
+      options:
+        name: George
+```
+Furthermore, every level of the command name heirarchy may contain options. For example, to define a configuration value for the option `--dir` for any command that begins with `my:`:
+```
+command:
+  my:
+    options:
+      dir: '/base/path'
+    foo:
+      options:
+        name: George
+    bar:
+      options:
+        priority: high
+```
+
+### Configuration Value Substitution
+
+It is possible to define values in a configuration file that will be substituted in wherever used. For example:
+```
+common:
+  path: '/shared/path'
+command:
+  my:
+    options:
+      dir: '${common.path}'
+    foo:
+      options:
+        name: George
+```
+
+[grasmash/yaml-expander](https://github.com/grasmash/yaml-expander) is used to provide this capability.
+
 ## API Usage
 
+The easiest way to utilize the capabilities of this project is to use [Robo as a framework](https://robo.li/framework) to create your commandline tools. Using Robo is optional, though, as this project will work with any Symfony Console application.
+
 ### Load Configuration Files
 ```
 use Consolidation\Config\Config;
@@ -39,6 +93,18 @@ $processor->extend($loader->load('defaults.yml'));
 $processor->extend($loader->load('myconf.yml'));
 $config->import($processor->export());
 ```
+
+### Set Up Command Option Configuration Injection
+
+```
+$configInjector = new InjectConfigForCommand($config);
+$application = new Application($name, $version);
+
+$eventDispatcher = new \Symfony\Component\EventDispatcher\EventDispatcher();
+$eventDispatcher->addSubscriber($configInjector);
+$application->setDispatcher($eventDispatcher);
+```
+
 ### Get Configuration Values
 If you have a configuration file that looks like this:
 ```