Update documentation

Author: daniel-zahariev

README.md

@@ -8,9 +8,85 @@
 
 An easy to use **SDK** for **PHP** with all the best practices to kickstart your integration with the **Settle Payment Platform**.
 
-THIS IS A DRAFT
-
 ## Installation
 
 `composer require danielz/settle-sdk-php`
 
+## Usage
+
+The library provides a basic Client class that handles the communication with the Settel REST API:  
+
+```php
+$settle_client = new SettleApiClient(
+    SETTLE_MERCHANT_ID,
+    SETTLE_USER_ID,
+    SETTLE_PUBLIC_KEY,
+    SETTLE_PRIVATE_KEY,
+    SETTLE_IN_SANDBOX
+);
+```
+
+The library then provides an entry point class for each section in the [REST API](https://api.support.settle.eu/api/reference/rest/v1/).
+Currently, only MerchantsApi is available as Permissions and OAuth2 are pending breaking changes and will be added soon.
+
+
+### Merchants API
+The following class serves as an entry point for all the resources in the section:  
+```php
+$merchant_api = new MerchantApi($settle_client);
+```
+Each resource (class) can be accessed via magic property on the Merchants API object:
+
+```php
+$merchant_api->api_keys->...;
+$merchant_api->balance->...;
+$merchant_api->dynamic_links->...;
+$merchant_api->payment_requests->...;
+$merchant_api->payment_sends->...;
+$merchant_api->pos->...;
+$merchant_api->profile->...;
+$merchant_api->settlements->...;
+$merchant_api->short_links->...;
+$merchant_api->status_codes->...;
+```
+
+The methods that each class implements very closely matches the REST API specification:
+
+```php
+$merchant_api->api_keys->list();
+$merchant_api->api_keys->get($api_key_id);
+$merchant_api->api_keys->create($api_key_data);
+$merchant_api->api_keys->update($api_key_id, $api_key_data);
+$merchant_api->api_keys->delete($api_key_id);
+```
+
+Only `PaymentRequests` class has a few extra helper methods:
+
+```php
+$merchant_api->payment_requests->capture($payment_request_id,$currency,$amount)
+$merchant_api->payment_requests->refund($payment_request_id, $currency, $amount);
+$merchant_api->payment_requests->getPaymentLink($payment_request_id);
+$merchant_api->payment_requests->getMobilePaymentLink($payment_request_id);
+```
+
+
+
+
+
+
+
+### Webhooks / Callbacks
+In order to validate callback requests from Settle, both Apache and nginx servers require manual setup to pass the `Authorization` header to PHP. 
+
+Here's an example for Apache:
+
+```
+RewriteEngine On
+RewriteRule .* - [e=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
+```
+
+Here's an example for nginx:
+```
+proxy_set_header Authorization $http_authorization;
+proxy_pass_header  Authorization;
+```