Draft up readme with some helper use cases.

readme.md

@@ -1,5 +1,144 @@
 # Laravel ETLs
 
+Define ETL pipelines to extract, transform, and load data from one source into your Laravel application.
+
+# Installation
 ```php
 composer require ziffmedia/laravel-etls
-```
+```
+
+# Development
+## Configuration
+Upon installing this library, you'll first need to create a directory called `Etls` at `App/Etls`.
+Next, create a class file in this directory that extends the `AbstractEtl` class, which implements
+the required `extractor` and `loader` methods. The `extractor` method must return an object which
+implements the `Extractor` interface, and the `loader` method must return an object which implements
+the `Loader` interface. Here is an arbitrary example:
+
+```php
+<?php
+
+namespace App\Etls;
+
+use ZiffMedia\LaravelEtls\AbstractEtl;
+use ZiffMedia\LaravelEtls\Contracts\Extractor;
+use ZiffMedia\LaravelEtls\Contracts\Loader;
+use ZiffMedia\LaravelEtls\DbExtractor;use ZiffMedia\LaravelEtls\DbLoader;
+
+final class CustomersEtl extends AbstractEtl
+{
+    public function extractor(): Extractor
+    {
+        $extractor = new DbExtractor(app('db')->connection('external_db'));
+        $extractor->query()
+            ->select([
+                'username'
+                'email'
+            ])
+            ->from('users');
+
+        return $extractor;
+    }
+
+    public function loader(): Loader
+    {
+        $loader = new DbLoader(DB::connection());
+        $loader->table('customers')
+            ->columns([
+                'username',
+                'email'
+            ]);
+
+        return $loader;
+    }
+}
+```
+
+The class above will register an ETL with the name `customers` that can be run using
+Artisan. e.g., `artisan etls:run customers`. Please see the usage section below for additional details.
+
+## Transformation
+By default, the `AbstractEtl` class passes data from the `Extractor` to the `Loader` in the exact
+"shape" it extracted it in. To modify data before loading it into your Laravel app, you can override
+the `transform` method. Here's another arbitrary example:
+
+```php
+final class CustomersEtl extends AbstractEtl
+{
+    // ...other details from above
+
+    public function transform(array $data): array|Iterator|null
+    {
+        $transformedData = array_merge($data, ['email_address' => $data['email']]);
+
+        unset($transformedData['email']);
+
+        return $transformedData;
+    }
+}
+```
+
+The above example would assume you're loading the `email` column from the data source into an
+`email_address` field in the data destination. Naturally, real transformations will usually be more
+involved.
+
+## Key Mapping
+By default, laravel-etls will look for an `id` field in both the data source and the destination
+as the unique key to keep track of. Sometimes, those values are not the same type. For instance,
+you might have an `id` of type `string` (e.g., a UUID) in the source destination, and an `id` of type
+`int` in the destination. For cases like these, you'll want to ensure you have a legacy key set up
+in your destination database so the library can know how to correctly map that data on subsequent
+runs of the ETL. For this, you'll want to use the `uniqueColumns` method. Here's an arbitrary example:
+
+```php
+<?php
+
+namespace App\Etls;
+
+use ZiffMedia\LaravelEtls\AbstractEtl;
+use ZiffMedia\LaravelEtls\Contracts\Extractor;
+use ZiffMedia\LaravelEtls\Contracts\Loader;
+use ZiffMedia\LaravelEtls\DbExtractor;use ZiffMedia\LaravelEtls\DbLoader;
+
+final class CustomersEtl extends AbstractEtl
+{
+    public function extractor(): Extractor
+    {
+        $extractor = new DbExtractor(app('db')->connection('external_db'));
+        $extractor->query()
+            ->select([
+                'id AS legacy_id',
+                'username'
+                'email'
+            ])
+            ->from('users');
+
+        return $extractor;
+    }
+
+    public function loader(): Loader
+    {
+        $loader = new DbLoader(DB::connection());
+        $loader->table('customers')
+            ->columns([
+                'username',
+                'email'
+            ])
+            ->uniqueColumns(['legacy_id']);
+
+        return $loader;
+    }
+}
+```
+
+# Usage
+```shell
+# Display the number of records from extractor and loader index for a given ETL.
+artisan etls:info <name-of-your-etl>
+
+# List the available ETLs to run.
+artisan etls:list
+
+# Run a specified ETL.
+artisan etls:run <name-of-your-etl>
+```