update readme

Author: marianobrc

README.md

@@ -27,6 +27,26 @@ DOWNLOADER_MIDDLEWARES = {
     'scrapy_proxycrawl.ProxyCrawlMiddleware': 610
 }
 ```
+## Usage
+
+Use the scrapy_proxycrawl.ProxyCrawlRequest instead of the scrapy built-in Request:
+
+```python
+from scrapy_proxycrawl import ProxyCrawlRequest
+
+yield ProxyCrawlRequest(
+    url,
+    callback=self.parse_result
+    device='desktop',
+    country='US',
+    page_wait=8000,
+    ajax_wait=True,
+    dont_filter=True
+)
+```
+The scrapy_proxycrawl.ProxyCrawlRequest accepts additional arguments, used in Proxy Crawl API.
+The request and response will be handled by the middleware, switching the urls automatically.
+
 
 If you have questions or need help using the library, please open an issue or [contact us](https://proxycrawl.com/contact).