Add documentation

Author: mnapoli

README.md

@@ -22,4 +22,49 @@ Requirements:
 
 ## Usage
 
-TODO
+The kernel's role is to create the container. It does so by registering all the configuration files of the modules we ask it to load:
+
+```php
+$kernel = new Kernel([
+    'twig',
+    'doctrine',
+    'app',
+]);
+
+$container = $kernel->createContainer();
+```
+
+### Installing a module
+
+To install a 3rd party module:
+
+- install the package using Composer
+- add it to the list of modules your kernel will load, for example:
+
+    ```php
+    $kernel = new Kernel([
+        'twig',
+    ]);
+    ```
+
+### Creating a module
+
+1. choose a module name, for example `blogpress`
+1. create a resource directory in your package, usually `res/`
+1. map it with Puli, for example `puli map /blogpress res`
+1. create as many PHP-DI configuration files as needed in `res/config/`
+
+That's it. Here is what your package should look like:
+
+```
+res/
+    config/
+        config.php
+    ...
+src/
+    ...
+composer.json
+puli.json
+```
+
+When users install your package and tell the kernel to load the `blogpress` module, it will load all the files matching the Puli path `/blogpress/config/*.php` (i.e. `vendor/johndoe/blogpress/res/config/*.php` on the filesystem).