Add example of posting plain/text

docs/Sending_Requests.fsx

@@ -92,6 +92,23 @@ http {
 *)
 
 
+(**
+## Plain text
+*)
+http {
+    POST "https://mysite"
+    body
+    // Sets Content-Type: plain/text header
+    text """
+The last train is nearly due
+The underground is closing soon
+And in the dark deserted station
+Restless in anticipation
+A man waits in the shadows
+"""
+}
+|> Request.send
+
 (**
 ## Request Cancellation