Complete the getting_started.md file

docs/getting_started.md

@@ -8,20 +8,45 @@ First make sure you have [installed Behat][1].
 Usage
 =====
 
-Refer to each adapter's documentation.
+The entrypoint of the library is the [Presta\BehatParser\Evaluator][2] class.
+
+Invoke it with an input value and retrieve the value after it has been evaluated and possibly modified by each registered [Presta\BehatParser\Adapter\AdapterInterface][3] sequentially.
+
+```php
+<?php
+
+use Presta\BehatParser\Adapter\AdapterInterface;use Presta\BehatParser\Evaluator;
+
+/** @var list<AdapterInterface> $adapters */
+$adapters = [
+    // ...
+];
+
+$evaluate = new Evaluator($adapters);
+$value = $evaluate('any value');
+```
 
 How it works?
 =============
 
-TODO
+The [Presta\BehatParser\Evaluator][2] takes a collection of [Presta\BehatParser\Adapter\AdapterInterface][3] which will be invoked sequentially to evaluate the input value.
+
+Each adapter is responsible to decide whether or not it should modify the input value.
+Internally, they may use simple PHP functions, regexes or even the [Symfony Expression Language][4] component to evaluate and modify the input value.
+
+In the end, the value will be returned after having been modified (or not) by one or several adapters.
 
 That was it!
 ============
 
 ---
 
-You may return to the [README.md][2] or read other [adapters documentation files][3].
+You may return to the [README.md][5] or refer to the [evaluator][6] or the [adapters][7] documentation files.
 
 [1]: https://behat.org/en/latest/quick_start.html#installation
-[2]: ../README.md
-[3]: ./adapters/
+[2]: ../src/Evaluator.php
+[3]: ../src/Adapter/AdapterInterface.php
+[4]: https://symfony.com/doc/current/components/expression_language.html
+[5]: ../README.md
+[6]: ./evaluator.md
+[7]: ./adapters