Add documentation for the SessionStorageService class

Author: Ingannatore

README.md

@@ -22,6 +22,10 @@ php composer.phar require gibilogic/elements
 
 Here you can find an overview for all the elements of our library. 
 
+### Session storage service
+
+A simple key-based session storage service for arrays made of key-value pairs. [Read its documentation](Resources/doc/session_storage_service.md).
+
 ### Slugger service 
 
 A simple slugger, used to "sanitize" strings by removing bizarre and URL-unfriendly characters. [Read its documentation](Resources/doc/slugger_service.md).

Resources/doc/session_storage_service.md

@@ -0,0 +1,65 @@
+# Session storage service
+
+This service helps when dealing with key-value arrays that must be saved indipendently inside the session.
+
+The classic use-case is when two or more CRUD-based entity lists have session-persisted filters: each list has its own key-value filters and they must be managed indipendently.
+
+## Symfony configuration
+
+If you want to use this service as a Symfony service, add the following configuration to your `services.yml` file:
+
+```yaml
+services:
+    # Other services...
+    gibilogic.session_storage:
+        class: Gibilogic\Elements\Service\SessionStorageService
+        arguments:
+            - "@session"
+```
+
+You'll be able to get an instance of the service from the service container:
+
+```php
+/* @var \Gibilogic\Elements\Service\SessionStorageService $sessionStorageService */
+$sessionStorageService = $this->container->get('gibilogic.session_storage');
+```
+
+## Usage
+
+With this service you can `set`, `get` and `remove` key-value arrays by giving them a name (aka. "key").
+
+For setting a key-value array use the `set` method:
+
+```php
+$sessionStorageService->set('order', [
+    'fromDate' => '2016-01-01',
+    'toDate' => '2016-02-01',
+]);
+```
+
+Getting the data back is as simple as calling the `get` method:
+
+```php
+$filters = $sessionStorageService->get('order');
+```
+
+Call the `remove` method to clear your data:
+
+```php
+$sessionStorageService->remove('order');
+```
+
+### Setting with overwrite
+
+By default the `set` method will use an `array_replace` to save your data: existing data will be merged by replace with the new data.
+
+If you want to disable this feature, just pass a boolean `true` as second parameter to the `set` method: 
+
+```php
+$sessionStorageService->set('order', [
+    'fromDate' => '2016-01-01',
+    'toDate' => '2016-02-01',
+], true);
+```
+
+The default behaviour can be useful when dealing with CRUD/APIs filters: you may want to update the changed values only, keeping the unchanged ones.