[Docs] add registration/login examples

docs/getting-started-with-sylius/using-api.rst

@@ -0,0 +1,64 @@
+Using API
+=========
+
+Since sylius 1.8 we allow you to use our API.
+Sylius API is based on ApiPlatform.
+Here are some examples of basic usage for a shop implementation.
+
+Registration
+------------
+
+To register a customer all we need to do is single post request:
+
+.. code-block:: bash
+
+    curl -X 'POST' \
+        'https://master.demo.sylius.com/api/v2/shop/customers' \
+        -H 'accept: */*' \
+        -H 'Content-Type: application/ld+json' \
+        -d '{
+        "firstName": "shop",
+        "lastName": "user",
+        "email": "shop.user@example.com",
+        "password": "pa$$word",
+        "subscribedToNewsletter": true
+    }'
+
+If we get response code `204`, it means our customer is registered successfully.
+
+
+Login
+-----
+
+Once shop customer is registered, we can now create a login request to get authentication token, which will allow us to use more shop endpoints.
+To get token, lets create a simple login request:
+
+.. code-block:: bash
+
+    curl -X 'POST' \
+        'https://master.demo.sylius.com/api/v2/shop/authentication-token' \
+        -H 'accept: application/json' \
+        -H 'Content-Type: application/json' \
+        -d '{
+        "email": "shop.user@example.com",
+        "password": "pa$$word"
+    }'
+
+As a response we should get code `200`, along with `token` and `customer` iri.
+
+.. code-block:: json
+
+    {
+        "token": "string",
+        "customer": "iri"
+    }
+
+With token we can add it to header requests and authenticate ourselves to use full potential of sylius API.
+
+.. code-block:: bash
+
+    curl -X 'METHOD' \
+        'api-url' \
+        -H 'accept: application/ld+json' \
+        -H 'Authorization: Bearer string
+