curl: explain -d example (#4183)

pages/common/curl.md

@@ -16,7 +16,7 @@
 
 `curl -O -L -C - {{http://example.com/filename}}`
 
-- Send form-encoded data (POST request of type `application/x-www-form-urlencoded`):
+- Send form-encoded data (POST request of type `application/x-www-form-urlencoded`). Use `-d @file_name` or `-d @'-'` to read from STDIN:
 
 `curl -d {{'name=bob'}} {{http://example.com/form}}`