Added example of multipart/form-data

Author: myroslav

docs/source/upload.rst

@@ -22,3 +22,14 @@ HTTPie example::
 
     http -f POST http://api-sandbox.openprocurement.org/api/0/tenders/f6882fa63d5141bcabec54a4766eec61/documents file@page.pdf
 
+The request itself should look like::
+
+    POST /api/0.2/tenders/f6882fa63d5141bcabec54a4766eec61/documents HTTP/1.1
+    Content-Type: multipart/form-data; boundary=28e02f7d4a3c4da19c4e2589329ad36f
+    Host: api-sandbox.openprocurement.org
+
+    --28e02f7d4a3c4da19c4e2589329ad36f
+    Content-Disposition: form-data; name="file"; filename="page.pdf"
+
+    ..Contents of PDF goes here...
+    --28e02f7d4a3c4da19c4e2589329ad36f--